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This paper demonstrates the performance of our previously suggested property-energy consistent method on the example of the generation of effective basis sets, pecS-1 and pecS-2, suited for the calculation of hydrogen, carbon, nitrogen, and oxygen chemical shifts. The new basis sets were successfully approbated in the GIAO-DFT calculations of the chemical shifts of 35 molecules using six different functionals. The pecS-1 basis set demonstrated very good accuracy, which makes this small basis set an effective means for the large-scale computations. At the same time, the pecS-2 basis set also gave very accurate results, thus putting it on a par with the other commensurate basis sets suited for the chemical shifts calculations. [ABSTRACT FROM AUTHOR]

The quest for "chemical accuracy" is becoming more and more demanded in the field of structure and kinetics of molecules at solid surfaces. In this paper, as an example, we focus on the barrier for hydrogen diffusion on a α-Al2O3(0001) surface, aiming for a couple cluster singles, doubles, and perturbative triples [CCSD(T)]-level benchmark. We employ the density functional theory (DFT) optimized minimum and transition state structures reported by Heiden, Usvyat, and Saalfrank [J. Phys. Chem. C 123, 6675 (2019)]. The barrier is first evaluated at the periodic Hartree–Fock and local Møller–Plesset second-order perturbation (MP2) level of theory. The possible sources of errors are then analyzed, which includes basis set incompleteness error, frozen core, density fitting, local approximation errors, as well as the MP2 method error. Using periodic and embedded fragment models, corrections to these errors are evaluated. In particular, two corrections are found to be non-negligible (both from the chemical accuracy perspective and at the scale of the barrier value of 0.72 eV): the correction to the frozen core-approximation of 0.06 eV and the CCSD(T) correction of 0.07 eV. Our correlated wave function results are compared to barriers obtained from DFT. Among the tested DFT functionals, the best performing for this barrier is B3LYP-D3. [ABSTRACT FROM AUTHOR]

In this paper, equilibrium properties of structure II hydrates of hydrogen were determined from Monte Carlo simulations in the isothermal-isobaric Gibbs ensemble. Water and hydrogen molecules are described by the TIP4P/Ice and Silvera-Goldman models, respectively. The use of the Gibbs ensemble has many key advantages for the simulation of hydrates. By the separation of hydrogen vapor and hydrate phases into their own domains, coupled with transfer moves of hydrogen molecules between domains, cage occupancies were determined. Furthermore, the choice of this ensemble also allows equilibrium lattice constants and guest molecule chemical potentials to be straightforwardly estimated. Results for hydrogen mass fractions indicate reasonable agreement with prior simulation data and theoretical models, while detailed analysis of cage occupancy distributions and neighboring cage pair occupancy combinations gives valuable insight into the behavior of this hydrate at the inter-cage scale. These results will aid in the construction of theoretical models, for which knowledge of the occupancy of neighboring cages is of great importance. In support of previous experimental and theoretical works, we also find evidence of double occupancy of a few small cages inside of the hydrate stability zone, albeit at very high pressures; approximately 0.1% of small cages are doubly occupied at 300 MPa, for temperatures of 225 K and 250 K. [ABSTRACT FROM AUTHOR]

The manipulation of spin states at the molecular scale is of fundamental importance for the development of molecular spintronic devices. One of the feasible approaches for the modification of a molecular spin state is through the adsorption of certain specific atoms or molecules including H, NO, CO, NH3, and O2. In this paper, we demonstrate that the local spin state of an individual iron phthalocyanine (FePc) molecule adsorbed on an Au(111) surface exhibits controllable switching by hydrogen adsorption, as evidenced by using first-principles calculations based on density functional theory. Our theoretical calculations indicate that different numbers of hydrogen adsorbed at the pyridinic N sites of the FePc molecule largely modify the structural and electronic properties of the FePc/Au(111) composite by forming extra N-H bonds. In particular, the adsorption of one or up to three hydrogen atoms induces a redistribution of charge (spin) density within the FePc molecule, and hence a switching to a lowspin state (S = 1/2) from an intermediate spin state (S = 1) is achieved, while the adsorption of four hydrogen atoms distorts the molecular conformation by increasing Fe-N bond lengths in FePc and thus breaks the ligand field exerted on the Fe 3d orbitals via stronger hybridization with the substrate, leading to an opposite switching to a high-spin state (S = 2). These findings obtained from the theoretical simulations could be useful for experimental manipulation or design of single-molecule spintronic devices. [ABSTRACT FROM AUTHOR]

In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH4→ H2 + CH3. The present calculations were carried out using a modified and recalibrated version of the Jordan-Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom-diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. [ABSTRACT FROM AUTHOR]

Infrared-visible sum frequency generation (SFG) has seen increasing usage as a surface probe, particularly for liquid interfaces since they are amenable to few alternate probes. Interpreting the SFG data to arrive at a molecular-level configuration on the surface, however, remains a challenge. This paper reports a technique for analyzing and interpreting SFG data—called polarization-angle null or PAN-SFG. PAN-SFG enables ready identification of the ratio of the surface tangential and longitudinal hyperpolarizabilities—the hyperpolarizability direction—as well as the phase relationship between these components separated from the optical factors due to the substrate and experimental geometry. Separation of the surface optical factors results in an immediate connection between the null angle and the surface species polarization. If the Raman polarizability is also known, then PAN-SFG analysis, like the previously reported null techniques, provides a very accurate orientation. In addition, the reported polarization-angle, phase-shift analysis enables facile separation of the nonresonant background polarization from that of the resonant signal. Beyond orientation, PAN-SFG can be used to deconvolute overlapping resonances and identify components beyond a dipole response. This paper reports PAN-SFG for two systems providing deeper insight into both. An acetonitrile-water mixture was previously reported to undergo a phase transition at 7 mol %, attributed to a sudden change in orientation. PAN-SFG demonstrates that acetonitrile generates a classic dipole response and provides compelling evidence that the acetonitrile configuration remains constant as a function of concentration. An alternate model for the phase transition is presented. Like many aqueous systems, the SFG spectrum of the hydrogen-bonded region of ice consists of broad and overlapping features; features previously identified with PAN-SFG. Here PAN-SFG analysis is used to show that the reddest of these, the feature at 3098 cm-1, contains a significant quadrupole contribution that grows as the temperature is lowered. The quadrupole and its temperature dependence are used to assign the 3098 cm-1 feature to bilayer-stitching-hydrogen bonds. This is the first definitive assignment in the hydrogen-bonded region of water. [ABSTRACT FROM AUTHOR]

The scope of this paper is the theoretical study of hydrogen atom interaction with the boron-doped graphite surface taken as a model for the interactions that occur in controlled thermonuclear fusion devices. This work is carried out in the framework of the density functional theory. The boron-doped graphite surfaces are modeled using a small modified C[sub 16]H[sub 10] cluster, in which one or two carbon atoms are substituted by boron. The efficiency of the C[sub 16]H[sub 10] cluster in modeling the H-graphite interaction has already been established in a previous paper [J. Chem. Phys. 116, 8124 (2002)]. In this study, we show that the boron atom: (i) is not a stable adsorption site for H, that it induces (ii) an increase in the H binding energy, (iii) an increase in the permeability to H of the boron-doped graphite layer, and (iv) a long range electronic perturbation in its graphitic environment. A good agreement is found between our results and experimental studies dealing with erosion mechanisms of boron-doped graphite exposed to incident hydrogen ions fluxes. [ABSTRACT FROM AUTHOR]

In a recent paper, standard hard-sphere variational theory has been applied to pressure dissociation in dense fluid hydrogen (Juranek and Redmer, J. Chem. Phys. 112, 3780 (2000)). The correlation contributions to the dissociation equilibrium were determined from the free energy functional using effective pair potentials and minimization with respect to the hard-sphere reference system. For simplicity, the Berthelot mixing rule was used to determine the H-H[sub 2] pair potential (which entails additive effective hard spheres), and single-component reference pair correlation functions were employed for evaluating the correlation integrals. In this paper, we employ multicomponent reference pair correlations, and we study the sensitivity of the results with respect to nonadditivity of the effective hard spheres. We compare our results with available ab initio simulation data. [ABSTRACT FROM AUTHOR]

Three-dimensional quantum mechanical calculations have been carried out, using a modification of the log-derivative version of Kohn’s variational principle, to study the dissociation of HO2 into H and O2. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO2 is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption-type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O2 show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose in this paper a quantitative theoretical model to describe the recombination dynamics of hydrogen on Si(100)-(2×1) surface. The desorption kinetics of hydrogen on Si(100) has been experimentally determined to obey a first-order rate law and the internal state distributions of desorbed hydrogen has recently been determined experimentally using the resonantly enhanced multiphoton ionization technique. In this theoretical model, which has the characteristic of preassociative desorption, the rate of desorption and the internal state distribution of H2 is given by a thermally averaged golden-rule expression. In particular, the desorption of H2 is supposed to result from a bound-free transition between an initially bound state composed of two H–Si dangling bonds on the same silicon dimer and a final continuum state consisting of H2 plus Si surface. In addition to explaining the first-order desorption kinetics, our model dictates that H2 will be vibrationally hot upon desorption, whereas rotation of H2 will generally be expected to be cold because of symmetry constraints. These conclusions about the dynamics are consistent with recent experiments [K. W. Kolasinski, S. F. Shane, R. N. Zare, J. Chem. Phys. 95, 5482 (1991); 96, 3995 (1992)] in which hydrogen is found to be vibrationally excited but rotationally cold. We show, in this paper, that essentially all recent experimental results on hydrogen desorption on Si(100) can be qualitatively explained based on the Franck–Condon factors in our model. A co-planar model calculation is carried out using our method, and the calculated rovibrational distribution of H2 is compared with the aforementioned experimental results of Kolasinski, Shane, and Zare. [ABSTRACT FROM AUTHOR]

High resolution ir data on weakly bound OCOHF complexes formed in a slit supersonic expansion reveal a progression of extremely low frequency vibrational levels associated with the bending of the OCO–HF hydrogen bond. In a previous paper [J. Chem. Phys. 93, 7716 (1990)], we presented a spectroscopic analysis of the fundamental, combination and hot bands observed, corresponding to transitions between vlbend=00, 11, 20, 22, and 31, where vlbend denotes quanta of OCOHF skeletal bend excitation with l units of vibrational angular momentum. In this paper, we analyze the rotationally resolved data in terms of the rigid bender formalism of Hougen, Bunker and Johns to determine an explicit angular potential, V(θ), for the OCOHF complex in both the HF ground (vHF=0) and vibrationally excited (vHF=1) state.The OCOHF ground state (vHF=0) potential is dominated by quartic and sextic angular terms, and thus is surprisingly shallow with respect to the bending angle. This quasilinear vibrational behavior is characterized by wide amplitude bending wave functions with zero point motion extending from -38° to +38°. In contrast, the OCOHF excited state (vHF=1) exhibits a significantly bent equilibrium geometry with a hydrogen bond bend angle of 31°±5°, corresponding to a cylindrically symmetric, noncolinear minimum in the potential. This shift in equilibrium geometry upon vHF excitation is quantitatively responsible for promoting Δvbend=0,2,... combination band vibrational modes, in analogy with Franck–Condon progressions in a bent←linear electronic transition.The predissociation lifetimes for vHF=1 excited OCOHF vary systematically with vlbend, and can be analyzed in terms of a geometry dependent predissociation rate which increases with bending of the hydrogen bond angle. These empirical bending potentials are in qualitative agreement with, but quantitatively much shallower than predicted by previous electrostatic and ab... [ABSTRACT FROM AUTHOR]

A previous calculation for H+N2 [Walch, Duchovic, and Rohlfing, J. Chem. Phys. 90, 3230 (1989)] focused on the minimum energy path (MEP) region of the potential energy surface and on estimates of the lifetime of the HN2 species. In this paper, we report energies computed at geometries selected to permit a global representation of the potential energy surface (PES). As in the previous work, the calculations were performed using the complete active space self-consistent field/externally contracted configuration interaction (CASSCF/CCI) method. The surface was characterized using the same basis set as in the previous paper except that an improved contraction of the H s basis is used. Calculations with a larger basis set were carried out along an approximate MEP obtained with the smaller basis set. The new PES exhibits a sharp curvature, which was not present in the previous calculations, and has a slightly narrower and smaller barrier to dissociation. Saddle points for H atom exchange via collinear and T-shaped HN2 complexes are also reported. [ABSTRACT FROM AUTHOR]

The solution of the reactive scattering problem by R-matrix propagation requires a number of choices which may affect the numerical accuracy. Although the formal definitions and development were given in detail in the preceding paper, in this paper we give (a) the specific choices and numerical procedures followed in our calculation, (b) a discussion and evaluation of the stability of the calculation with respect to these choices, and (c) results for three dimensional J=0 reactive scattering of H3, H2D, and HD2 systems in the energy range from threshold to about 1.30 to 1.50 eV. [ABSTRACT FROM AUTHOR]

Methanol synthesis from CO and H2 over ZnO, which requires high temperatures and high pressures giving rise to a complex interplay of physical and chemical processes over this heterogeneous catalyst surface, is investigated using ab initio simulations. The redox properties of the surrounding gas phase are known to directly impact on the catalyst properties and thus, set the overall catalytic reactivity of this easily reducible oxide material. In Paper III of our series [J. Kiss, J. Frenzel, N. N. Nair, B. Meyer, and D. Marx, J. Chem. Phys. 134, 064710 (2011)] we have qualitatively shown that for the partially hydroxylated and defective ZnO(0001) surface there exists an intricate network of surface chemical reactions. In the present study, we employ advanced molecular dynamics techniques to resolve in detail this reaction network in terms of elementary steps on the defective surface, which is in stepwise equilibrium with the gas phase. The two individual reduction steps were investigated by ab initio metadynamics sampling of free energy landscapes in three-dimensional reaction subspaces. By also sampling adsorption and desorption processes and thus molecular species that are in the gas phase but close to the surface, our approach successfully generated several alternative pathways of methanol synthesis. The obtained results suggest an Eley-Rideal mechanism for both reduction steps, thus involving "near-surface" molecules from the gas phase, to give methanol preferentially over a strongly reduced catalyst surface, while important side reactions are of Langmuir-Hinshelwood type. Catalyst re-reduction by H2 stemming from the gas phase is a crucial process after each reduction step in order to maintain the catalyst's activity toward methanol formation and to close the catalytic cycle in some reaction channels. Furthermore, the role of oxygen vacancies, side reactions, and spectator species is investigated and mechanistic details are discussed based on extensive electronic structure analysis. [ABSTRACT FROM AUTHOR]

In this paper, we describe the analysis of the thermodynamic properties of cryogenic hydrogen using classical molecular dynamics (MD) and path integral MD (PIMD) method to understand the effects of the quantum nature of hydrogen molecules. We performed constant NVE MD simulations across a wide density-temperature region to establish an equation of state (EOS). Moreover, the quantum effect on the difference of molecular mechanism of pressure-volume-temperature relationship was addressed. The EOS was derived based on the classical mechanism idea only using the MD simulation results. Simulation results were compared with each MD method and experimental data. As a result, it was confirmed that although the EOS on the basis of classical MD cannot reproduce the experimental data of saturation property of hydrogen in the high-density region, the EOS on the basis of PIMD well reproduces those thermodynamic properties of hydrogen. Moreover, it was clarified that taking quantum effects into account makes the repulsion force larger and the potential well shallower. Because of this mechanism, the intermolecular interaction of hydrogen molecules diminishes and the virial pressure increases. [ABSTRACT FROM AUTHOR]

Tensor hypercontraction is a method that allows the representation of a high-rank tensor as a product of lower-rank tensors. In this paper, we show how tensor hypercontraction can be applied to both the electron repulsion integral tensor and the two-particle excitation amplitudes used in the parametric 2-electron reduced density matrix (p2RDM) algorithm. Because only O(r) auxiliary functions are needed in both of these approximations, our overall algorithm can be shown to scale as O(r4), where r is the number of single-particle basis functions. We apply our algorithm to several small molecules, hydrogen chains, and alkanes to demonstrate its low formal scaling and practical utility. Provided we use enough auxiliary functions, we obtain accuracy similar to that of the standard p2RDM algorithm, somewhere between that of CCSD and CCSD(T). [ABSTRACT FROM AUTHOR]

Hydrogen gas has been detected in a closed system containing copper and pure anoxic water [P. Szakalos, G. Hultquist, and G. Wikmark, Electrochem. Solid-State Lett. 10, C63 (2007) and G. Hultquist, P. Szakalos, M. Graham, A. Belonoshko, G. Sproule, L. Grasjo, P. Dorogokupets, B. Danilov, T. Aastrup, G. Wikmark, G. Chuah, J. Eriksson, and A. Rosengren, Catal. Lett. 132, 311 (2009)]. Although bulk corrosion into any of the known phases of copper is thermodynamically forbidden, the present paper shows how surface reactions lead to the formation of hydrogen gas in limited amounts. While water cleavage on copper has been reported and investigated before, formation of molecular hydrogen at a single-crystal Cu[100] surface is here explored using density functional theory and transition state theory. It is found that although solvent catalysis seems possible, the fastest route to the formation of molecular hydrogen is the direct combination of hydrogen atoms on the copper surface. The activation free energy (▵Gs‡f) of hydrogen formation in condensed phase is 0.70 eV, which corresponds to a rate constant of 10 s-1 at 298.15 K, i.e., a relatively rapid process. It is estimated that at least 2.4 ng hydrogen gas could form per cm2 on a perfect copper surface. [ABSTRACT FROM AUTHOR]

In this paper we report transition frequencies and line strengths computed for H2O-H2 and compare with the experimental observations of [M. J. Weida and D. J. Nesbitt, J. Chem. Phys. 110, 156 (1999)]. To compute the spectra we use a symmetry adapted Lanczos algorithm and an uncoupled product basis set. Our results corroborate the assignments of Weida and Nesbitt and there is good agreement between calculated and observed transitions. Possible candidates for lines that Weida and Nesbitt were not able to assign are presented. Several other bands that may be observable are also discovered. Although all the observed bands are associated with states localized near the global potential minimum, at which H2O acts as proton acceptor, a state with significant amplitude near the T-shape secondary potential minimum at which H2O acts as proton donor is identified by examining many different probability density plots. [ABSTRACT FROM AUTHOR]

In the present paper, we investigate the molecular structure and hyperfine couplings of a series of σ radicals containing first- and second-row atoms (H2CN,H2CP,NH2,PH2) for which accurate gas-phase microwave results are available. The presence of α- and, especially, β-hydrogen atoms makes the evaluation of magnetic properties of these radicals particularly challenging. Geometrical parameters have been computed by the coupled-cluster ansatz in conjunction with hierarchical series of basis sets, thus accounting for extrapolation to the complete basis-set limit. Core correlation as well as higher excitations in the electronic-correlation treatment have also been taken into account. An analogous approach has been employed for evaluating hyperfine coupling constants with particular emphasis given to basis-set, correlation, and geometrical effects. The corresponding vibrational corrections, required for a meaningful comparison to experimental data, have also been investigated. The remarkable agreement with experiment confirms the reliability of the present computational approach, already validated for π radicals, thus establishing the way for setting up a benchmark database for magnetic properties. [ABSTRACT FROM AUTHOR]

A new electron-methanol molecule pseudopotential is developed and tested in the present paper. The formal development of the potential is based on quantum mechanical calculations on the electron-methanol molecule model in the static exchange approximation. The computational model includes a steep confining potential that keeps the otherwise unbound excess electron in the vicinity of the methanol molecule. Using the Phillips–Kleinman theorem we introduce a smooth pseudowave function of the excess electron with the exact eigenenergy and correct asymptotic behavior. The nonlocal potential energy operator of the model Hamiltonian is then replaced to a local potential that reproduces the ground-state properties of the excess electron satisfactorily. The pseudopotential is then optimized in an analytically simple functional form to fit this approximate local potential in conjunction with the point charges and the geometry of a classical, all-site methanol-methanol interaction potential. Of the adjustable parameters, the parameters for the carbon and the methyl hydrogen atoms are optimized, while those for the oxygen and the hydroxyl hydrogen are taken from a previous electron-water molecule pseudopotential. A polarization term is added to the potential a posteriori. The polarization parameters are chosen to reproduce the experimental position of the optical absorption spectrum of an excess electron in mixed quantum-classical molecular dynamics simulations. The energetic, structural and spectroscopic properties of the solvated electron in a methanol bath are simulated at 300 K and compared with previous solvated electron simulations and available experimental data. [ABSTRACT FROM AUTHOR]

This paper presents comparisons between molecular dynamics simulations (MDSs) and the Keilson and Storer (KS) model for collision-induced translational velocity changes in pure H2 at room temperature from four different points of view. The first considers various autocorrelation functions associated with the velocity. The second and third comparisons are made for the collision kernels and for the time evolutions of some conditional probabilities for changes in the velocity modulus and orientation. Finally, the evolutions, with density, of the half widths of the Q(1) lines of the isotropic Raman (1–0) fundamental band and of the (2–0) overtone quadrupole band are investigated. The results demonstrate that, while the KS approach gives a poor description of detailed velocity-to-velocity changes, it leads to accurate results for the correlation functions and spectral shapes, quantities resulting from large averages over the velocity. On the opposite, collision kernels derived from MDS lead to accurate predictions of all considered quantities. The results open promising perspectives for modeling of the spectral shapes of other systems. They also stress the value of direct calculations of speed-dependent broadening and shifting parameters from the intermolecular potential to avoid their determination from measured spectra and permit fully meaningful tests of the models. [ABSTRACT FROM AUTHOR]

A theoretical analysis of the hydrogen-terminated (100) surface of silicon leads to the identification of a new configuration, formed by a silylene center interacting with vicinal silicon dihydrides. This structure may be viewed as a metastable configuration of 2×1 (100) (SiH)2. Silylene can however be stabilized via interaction with water. The paper proposes that some of the anomalous features observed at the hydrogen-terminated or oxidized (100) Si can be attributed to silylene centers datively stabilized by oxo groups or to structures resulting from their decomposition. [ABSTRACT FROM AUTHOR]

The hydrogen [1,3]-sigmatropic shift in propene is predicted by the Woodward–Hoffman rules to occur by an antarafacial pathway, yet the lack of experimental evidence suggests that this pathway is not favorable. Two natural questions arise: (i) can the [1,3]-shift be made more favorable by a symmetry-forbidden multistep pathway, and (ii) can the energetics be influenced by a substituent on propene? As in many chemical reactions, describing the energetics of these reactions requires a balanced treatment of both single-reference and multireference electron correlations, and yet traditional wave function methods often excel in treating only one kind of correlation. An equitable description of correlation effects, however, can be achieved, at a cost similar to efficient single-reference methods, by computing the two-electron reduced density matrix (2-RDM) from the anti-Hermitian part of the contracted Schrödinger equation (ACSE) [D. A. Mazziotti, Phys. Rev. Lett. 97, 143002 (2006)]. As with the contracted Schrödinger equation, the indeterminacy of the ACSE is removed without the many-electron wave function by reconstructing the 3-RDM from the 2-RDM via cumulant theory [D. A. Mazziotti, Chem. Phys. Lett. 289, 419 (1998)]. In this paper we apply the ACSE to study sigmatropic shifts in both propene and acetone enolate while extending its formalism to treat doublet spin states. In the 6-311G** basis set the ACSE predicts the activation energy of the trimethylene-to-propene rearrangement to be 8.8 kcal/mol while multireference perturbation theory yields a smaller barrier of 2.2 kcal/mol and coupled cluster singles-doubles predicts a negative barrier. We further find that the [1,3]-shift in acetone enolate is more favorable by ≈30 kcal/mol than the [1,3]-shift in propene, which is consistent with a prior theoretical investigation as well as experimental observations of these shifts in 2-butanone enolate. [ABSTRACT FROM AUTHOR]

On a new potential energy surface (PES-2008) developed by our group (preceding paper), we performed an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 250–2000 K and a dynamics study using quasiclassical trajectory (QCT) and quantum-mechanical (QM) calculations at collision energies between 0.7 and 2.0 eV for the title reaction and isotopically substituted versions. Kinetically, the H+CH4 forward and reverse thermal rate constants reproduce the available experimental data, with a small curvature of the Arrhenius plot indicating the role of tunneling in this hydrogen abstraction reaction. Five sets of kinetic isotope effects are also calculated. In general, they reproduce the experimental information. Dynamically, we focused on the H+CD4 reaction because there are more experimental studies for comparison. Most of the available energy appears as product translational energy (55%–68%), with the HD product being vibrationally cold (v′=0,1) in agreement with experiment, although rotationally hotter than experiment. The reaction cross section is practically negligible at 0.7 eV and still small at 1.5 eV, reproducing the experimental evidence, although our values are smaller. The product angular distribution is analyzed using QCT and QM methods. While at low energies (0.7 eV) both the QCT and the QM calculations yield forward scattered CD3 product, i.e., a rebound mechanism, at high energy (1.2 eV) only the QM calculations reproduce the experiment. The agreement with this wide variety of kinetic and dynamic experimental data (always qualitative and in some cases quantitative) shows the capacity of the PES-2008 surface to describe the reaction system. [ABSTRACT FROM AUTHOR]

In recent years, sum-frequency generation (SFG) has been used to investigate numerous interfaces including aqueous interfaces. A longstanding challenge to interpretation of the SFG results, along with the related aqueous-solution infrared and Raman spectra, is a lack of connection between features in the broad hydrogen-bonded region and molecular-level interactions or configurations. This paper reports results of a newly developed polarization analysis of the generated sum-frequency signal as a function of wavelength both to deconvolute spectral resonances and to characterize the dynamic polarization associated with the resonances. Operationally, the polarization angle of the generated sum frequency is determined by identifying the null angle. The technique is hence termed polarization-angle null analysis or PAN. PAN applied to ice is very powerful; it reveals that the hydrogen-bonded region of the basal face of ice Ih contains at least five oscillators, each with a distinct polarization. The dynamic polarizability of the longest wavelength oscillator is nearly entirely transverse (perpendicular to the surface normal, i.e., in the surface plane); in contrast, the shortest wavelength oscillator is almost entirely longitudinal (along the surface normal). [ABSTRACT FROM AUTHOR]

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti–Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti–Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

It is shown in this paper that from the study of the induced infrared absorption spectra of homonuclear diatomic molecules solvated as impurities in a molecular quantum solid, it is possible to extract information about the rovibrational matrix elements of the multipole moments and polarizability of the embedded molecule. Theoretical expressions are derived for the integrated absorption coefficients of various multipole-field-induced double transitions involving guest-host pairs in a solid para-H2 matrix. The intensities of some of the quadrupole moment induced transitions involving the N2-para-H2 pair have been measured. From a comparison of the experimental and theoretical intensities, rovibrational matrix elements of the quadrupole moment of N2 are determined in its ground vibrational state. [ABSTRACT FROM AUTHOR]

Our previous study [J. Chem. Phys. 122, 224316 (2005)] presented the photoacoustic and action spectra of the V=2, 3, 4, and 5 manifolds of the C–H methyl stretching vibrations of propyne and their analysis in terms of a simplified joint local mode/normal mode model. In the current paper the C–H transition intensities were calculated using B3LYP/6-311++G(d,p) level of theory to obtain the dipole moment functions. The diagonalization of the vibrational Hamiltonian revealed new model parameters obtained by least square fitting of the eigenvalues to the action spectra band origins, while examining the correspondence between the calculated intensities and simulated band areas. The newly derived parameters predict well the band positions and the observed intensities, allowing new assignment of the features. The derived Hamiltonian was also used to obtain the overall temporal behavior of the C–H stretches as a result of the Fermi couplings and interactions with the bath states. These results indicate that any specificity attained by suitable excitation of the methyl C–H stretches is lost on picosecond time scale, primarily due to strong interactions with doorway states in the lower overtone and coupling with bath states in the region of the higher ones. [ABSTRACT FROM AUTHOR]

In this paper a perturbation-theory study of vibrational lifetimes for the bending and stretching modes of hydrogen adsorbed on a Si(100) surface is presented. The hydrogen-silicon interaction is treated with a semiempirical bond-order potential. Calculations are performed for H–Si clusters of different sizes. The finite lifetime is due to vibration-phonon coupling, which is assumed to be linear or bilinear in the phonon and nonlinear in the H–Si stretching and bending modes. Lifetimes and vibrational transition rates are evaluated with one- and two-phonon processes taken into account. Temperature effects are also discussed. In agreement with the experiment and previous theoretical treatment it is found that the H–Si (υs=1) stretching vibration decays on a nanosecond timescale, whereas for the H–Si (υb=1) bending mode a picosecond decay is predicted. For higher-excited vibrations, simple scaling laws are found if the excitation energies are not too large. The relaxation mechanisms for the excited H–Si stretching and the H–Si bending modes are analyzed in detail. [ABSTRACT FROM AUTHOR]

Despite intensive work there are still controversial issues about desorption and adsorption of hydrogen on Si(100)-(2×1). In particular, the relative importance of the various interdimer- and intradimer-desorption paths is not clear. Nanosecond-pulse-laser desorption data have been used to argue that the 4H interdimer path is important, while data from thermal-desorption time-of-flight measurements suggest a large translationally hot contribution which cannot arise from the 4H interdimer path. The observation of a translationally hot desorption fraction at low to medium coverage can be accounted for by including the 2H interdimer path in quantum dynamical calculations. In this paper we investigate this issue further and present evidence that supports the inclusion of the intradimer path. Specifically, our results show that the intradimer and 3H interdimer paths provide the major contributions to the translationally hot fraction in the desorbate. Our conclusions are based on density-functional calculations of hydrogen translational excitation, mean-field analysis of thermal-desorption experiments over a range of ramp rate, and Monte Carlo simulations of nanosecond-pulse-laser experiments. [ABSTRACT FROM AUTHOR]

Fundamental issues associated with the application of the nuclear-electronic orbital (NEO) approach to hydrogen transfer systems are addressed. In the NEO approach, specified nuclei are treated quantum mechanically on the same level as the electrons, and mixed nuclear-electronic wavefunctions are calculated with molecular orbital methods. The positions of the nuclear basis function centers are optimized variationally. In the application of the NEO approach to hydrogen transfer systems, the hydrogen nuclei and all electrons are treated quantum mechanically. Within the NEO framework, the transferring hydrogen atom can be represented by two basis function centers to allow delocalization of the proton vibrational wavefunction. In this paper, the NEO approach is applied to the [He-H-He]+ and [He-H-He]++ model systems. Analyses of technical issues pertaining to flexibility of the basis set to describe both single and double well proton potential energy surfaces, linear dependency of the hydrogen basis functions, multiple minima in the basis function center optimization, convergence of the number of hydrogen basis function centers, and basis set superposition error are presented. The accuracy of the NEO approach is tested by comparison to grid calculations for these model systems. [ABSTRACT FROM AUTHOR]

A numerical treatment suitable for the computational investigation of physisorption of molecular hydrogen on carbon nanostructures has not been sufficiently discussed. In this paper it is shown that results used as a reference are actually a product of poorly solved interactions and contaminated estimates with errors which would be of the order of 60%. Moreover, using ab initio molecular orbital theory, under the rigid monomer supermolecular approach, the physisorption energy of molecular hydrogen on graphene was reinvestigated. The graphene surface was modeled as a coronenelike (C24H12) graphene sheet. The basis set superposition error was corrected by means of the counterpoise method. The H2–H2 and H2-benzene interactions were examined, under systematic combinations of basis sets and correlation methods, including the aug-cc-pVQZ basis set and the coupled cluster correlation method with single, double, and noniterative triple excitations, searching for a numerical treatment with a reasonable trade-off between efficiency and accuracy. Asymmetrical modeling strategies, using diffusion augmented basis sets with preference for the adsorbate, were found to be effective. Also local modeling strategies, using more complete basis sets for the nearest atoms to the adsorbate than for the rest of the substrate, were considered. The aug-cc-pVTZ basis set for the adsorbate and for the nearest atoms to the adsorbate and the cc-pVTZ basis set for the rest of the cluster-modeled graphene, at the second-order Möller–Plesset perturbation theory correlation level, was selected as reference treatment. It was found that the physisorption energy of molecular hydrogen on graphene would be of the order of 0.06 eV, which would be 25% less than what has been previously published, though it would be sufficient to permit the storage of hydrogen physisorbed on carbon. To our knowledge this would be the most realistic theoretical estimate of the mentioned energy to date. [ABSTRACT FROM AUTHOR]

The three-dimensional hydrogen cyanide/isocyanide isomerization problem is taken as an example to present a general theory for computing the phase space structures which govern classical reaction dynamics in systems with an arbitrary (finite) number of degrees of freedom. The theory, which is algorithmic in nature, comprises the construction of a dividing surface of minimal flux which is locally a ''surface of no return.'' The theory also allows for the computation of the global phase space transition pathways that trajectories must follow in order to react. The latter are enclosed by the stable and unstable manifolds of a so-called normally hyperbolic invariant manifold (NHIM). A detailed description of the geometrical structures and the resulting constraints on reaction dynamics is given, with particular emphasis on the three degrees of freedom case. A procedure is given which uses these structures to compute orbits homoclinic to, and heteroclinic between, NHIMs. The role of homoclinic and heteroclinic orbits in global recrossings of dividing surfaces and transport in complex systems is explained. The complete description provided here is inherently one within phase space; it cannot be inferred from a configuration space picture. A complexification of the classical phase space structures to incorporate quantum effects is also discussed. The results presented here call into question certain assumptions routinely made on the global dynamics; this paper provides methods that enable one to understand and quantify the phase space dynamics of reactions without making such assumptions. [ABSTRACT FROM AUTHOR]

Gas-phase hydrogen atoms are accelerated towards metallic surfaces in their vicinity. As it approaches the surface, the velocity of an atom increases and this motion excites the metallic electrons, causing energy loss to the atom. This dissipative dynamics is frequently described as atomic motion under friction, where the friction coefficient is obtained from ab initio calculations assuming a weak interaction and slow atom. This paper tests the aforementioned approach by comparing to a real-time Ehrenfest molecular dynamics simulation of such a process. The electrons are treated realistically using standard approximations to time-dependent density functional theory. We find indeed that the electronic excitations produce a frictionlike force on the atom. However, the friction coefficient strongly depends on the direction of the motion of the atom: it is large when the atom is moving towards the cluster and much smaller when the atom is moving away. It is concluded that a revision of the model for energy dissipation at metallic surfaces, at least for clusters, may be necessary. [ABSTRACT FROM AUTHOR]

In this paper we perform a quantal density functional theory (Q-DFT) study of the hydrogen molecule in its ground state. In common with traditional Kohn–Sham density functional theory, Q-DFT transforms the interacting system as described by Schrödinger theory, to one of noninteracting fermions—the S system—such that the equivalent density, total energy, and ionization potential are obtained. The Q-DFT description of the S system is in terms of “classical” fields and their quantal sources that are quantum-mechanical expectations of Hermitian operators taken with respect to the interacting and S system wave functions. The sources, and hence the fields, are separately representative of all the many-body effects the S system must account for, viz. electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects. The local electron-interaction potential energy of each model fermion is the work done to move it in the force of a conservative effective field that is the sum of the individual fields. The Hartree, Pauli, Coulomb, and correlation-kinetic energy components of the total energy are also expressed in virial form in terms of the corresponding fields. The highest occupied eigenvalue of the S system is the negative of the ionization potential energy. The Q-DFT analysis of the hydrogen molecule is performed employing the highly accurate correlated wave function of Kolos and Roothaan. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper examines the resonances in H+HLi scattering. The signature of these resonances is obtained from the oscillations in its reaction probability versus energy curves. They are identified here from a set of pseudospectra calculated for different initial locations of a stationary Gaussian wave packet on the ab initio potential energy surface (PES) reported by Dunne, Murrel, and Jemmer [Chem. Phys. Lett. 336, 1 (2001)]. The nuclear motion on this PES is monitored with the aid of a time-dependent wave packet method and the pseudospectrum are calculated by Fourier transforming the time autocorrelation function of the initial wave packet. The resonances are further examined and assigned by computing their eigenfunctions through spectral quantization algorithm. Both the linewidth as well as decay lifetimes of the resonances are reported. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The speed and angular distributions of H atom products arising in the photodissociation of jet-cooled ketene (CH[sub 2]CO) molecules following excitation at 193.3, 203.3, 209, and 213.3 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. The observed product energy disposal is interpreted in terms of one photon absorption to the [sup 1]B[sub 1] electronically excited state, internal conversion to high lying vibrational levels of the ground state and subsequent unimolecular decay to yield the observed H (+HCCO) products. H atoms resulting from secondary photolysis of H containing primary products (most probably singlet CH[sub 2] radicals) are evident in the measured spectra, especially at high photolysis laser pulse energies. The kinetic energy distributions of the primary H+HCCO products span all energetically accessible product internal energies, peaking at ∼1170 cm-1 in the case of parent excitation at 213.3 nm, and rising to ∼1450 cm-1 (when exciting at 193.3 nm). These distributions are reproduced, qualitatively, by the statistical adiabatic product distribution (SAPD) method proposed recently by Cole and Balint-Kurti (J. Chem. Phys., preceding paper). This method is based on the use of a quantum mechanical, J conserving, Rice–Ramsperger–Kassel–Marcus (RRKM) treatment and provides a prediction of the product quantum state distributions and the total kinetic energy release spectra. Accurate, quadratic configuration interaction, intrinsic reaction coordinates have been computed for both the lowest singlet (S[sub 0]) and triplet (T[sub 1]) potential energy surfaces of CH[sub 2]CO. Quantum mechanical SAPD calculations have been performed using both surfaces; the results favor the conclusion that the dissociation occurs on the S[sub 0] surface. This conclusion is further supported by comparison of the calculated and previously measured CO product vibrational quantum state distributions arising from photodissociation at 193.3 nm. The variational RRKM method has also been used to compute the branching ratios for forming H+HCCO and CH[sub 2]+CO products on both the S[sub 0] and T[sub 1] surfaces. Different aspects of the SAPD model, such as the inclusion of quantum mechanical tunneling, the attractiveness of the long-range interfragment potential and the assumed adiabaticity of the fragmentation, have been varied in order to shed light on the nature of the dissociation process and the possible origins of the differences between the model calculations and the experimental results. It is found that the agreement between the quantum mechanical statistical model predictions and the experimentally observed total kinetic energy release spectra for the H atom dissociation channel can be greatly improved if the contribution of lower fragment relative orbital angular momenta is increased over that required by the use of a purely statistical model. This finding is equivalent to the conclusion that the dissociation is not entirely statistical, but that the dynamics of the break-up process plays some role. In particular the initial geometry of the parent molecule may restrict the body-fixed angles into which the final products can scatter and, through this, may restrict the relative orbital angular momenta to be on average smaller than that predicted by a purely statistical theory. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The metabolism of hydrogen (H[SUB2]↔2H[SUP+]+2e[SUP-) constitutes a central process in the global biological energy cycle. Among all the enzymes that can mediate this process, Fe-only hydrogenases are unique in their particular high reactivity. Recently, some important progresses have been achieved. Following our previous paper [Z.-P. Liu and P. Hu, J. Am. Chem. Soc. 124, 5175 (2002)] that characterizes the individual redox state of the active site of Fe-only hydrogenase, in this work we have determined the feasible reaction pathways and energetics for the H[SUB2] metabolism on the active site of Fe-only hydrogenases, using density functional theory. We show that H[SUB2] metabolism possesses very low reaction barriers and a proximal base from a nearby protein plays an important role in H[SUB2] metabolism. [ABSTRACT FROM AUTHOR]

This paper presents fully converged integral cross sections for the D[sub 2](j[sub 1]=0,2,4)+OH(j[sub 2]=0,2,4)→HOD+D reaction on the YZCL2 potential energy surface. It is found that the influence of reagent rotation excitation on reactivity is relatively mild compared with that of the D[sub 2] vibration excitation, but is fairly important to the thermal rate constant at low temperature. In general, the OH rotation excitation tends to hinder the reaction, and the D[sub 2] rotation excitation tends to enhance the reactivity. The simultaneous reagent rotation has some correlated effect, with the influence of the D[sub 2] rotation excitation considerably dependent on the OH rotation excitation. It is also found that the reaction is strongly dependent on j[sub 12] (the coupled angular momentum between j[sub 1] and j[sub 2]), K[sub 0] (the projection of the reagent rotation on the body fixed axis), and the parity of the system, in particular for either the OH or D[sub 2] rotationally excited initial states, but this dependence becomes relatively weaker as the collision energy increases. Excellent agreement achieved between theory and experiment on the thermal rate coefficients for temperatures up to 400 K clearly shows that the YZCL2 potential energy surface is capable of producing “quantitatively” accurate results for the title reaction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energy. The rotational orientation effect observed in the four dimensional studies remains in the present full dimensional dynamics with the cartwheel orientation yielding dramatically lower dissociative efficiency than the energetically equivalent helicopter orientation. We focus on normal incident scattering. The diffractive scattering and more detailed results will be presented in a later paper.. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations on the Muckerman V potential energy surface were carried out for the reaction F+HD(υ=0, J)→FH+D or FD+H. The reactive cross sections for the two product channels as well as the isotope effect were calculated for J=0 over the relative collision energy range 0.8–900 kcal/mol and also for 0≤J≤10 at a collision energy of 2 kcal/mol. The results are compared to those for F+H2→FH+H. The goal of this work was to understand the remarkable isotope effect behavior in both sets of calculations. It proved useful to decompose the reactive cross sections QR(FH) and QR(FD) into the product of three factors; the cross sections for F hitting the H end and the D end of HD, Qhit(FHD) and Qhit(FDH); the probabilities of reacting to give either FH or FD once the H end or D end is hit, PR(FHD) and PR(FDH); and the corrections for knockout collisions at both ends of the molecule, CKO(FHD) and CKO(FDH). (A knockout collision is one where the F atom initially hits the H end but reacts to form FD or vice versa.) All three factors make important contributions to the isotope effect. At low collision energies for J=0 FD is favored over FH because F hits the D end of HD more often and because the overall reactivity is higher at the D end. Both effects become less important at higher energies, but above 30 kcal/mol knockout reactions strongly favor the production of FD. The isotope effect as a function of J is quite remarkable. Although FD is favored for J=0 at E=2 kcal/mol, by J=3 no trajectories hit the D end, and no DF is formed. But at J=7 DF product reappears, coinciding with the onset of knockout reactions and an increase in reactivity at both ends of HD. We attribute all of these effects to the ability of the system at high J to rotate through the barrier separating the reactant valleys at each end of HD. The results obtained in this paper are explained by reference to special features of the potential... [ABSTRACT FROM AUTHOR]

HOCl is a closed shell system in its ground electronic state. In common with many molecules it is photodissociated to give open shell fragments. In this paper the photodissociation process HOCl(X 1A′)+hν→HOCl(2 1A′)→OH+Cl is investigated. A model is proposed to predict the OH product spin–orbit and Λ-doublet state distributions as well as the distribution of the Cl atom spin–orbit states. The model considers both the electronic structure of the system and the nuclear motion. Predictions are made not only of the individual fragment state distributions and of their dependence on total initial angular momentum and photon frequency, but also of the degree of correlation between the production of the different states of the two open shell fragments. The computed results agree with the available experiments, in particular the current theory is the first to agree with experimental findings in predicting a smoothly varying OH rotational state distribution. It is demonstrated that the model reproduces the frequency dependence of the absorption line shape as computed using more exact procedures. In the simple treatment presented here rotational transitions during the dissociation process are ignored, as is the detailed coupling of the electronic states in the asymptotic region. Some of the remaining disagreements between the current predictions and the experimental results are ascribed to these approximations in the model. It is hoped that the predictions of the correlated product state distributions will stimulate coincidence experiments on the photodissociation process. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This is the first of a series of papers in which we investigate the unimolecular dissociation of hydroperoxyl. Using the DMBE IV potential energy surface [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)], in the present study 726 bound states of HO2(X) up to the H+O2 dissociation threshold are calculated in an attempt to access the extent of the coupling between the modes of the system. The first approach involves an analysis of the nodal structure of the wave functions. While the wave functions for the lowest states are regular and assignable, the degree of mixing and complexity rapidly increases with energy. The wave functions close to the dissociation threshold are mostly irregular without any clear cut nodal structure and fill the entire coordinate space available. Nevertheless, a small number of regular states, that are associated with large excitation in the O2 stretching coordinate and no or only little excitation in the other modes, are found even at high energies. The second approach used to study the degree of intramolecular coupling is an analysis of the energy spectrum. The nearest neighbor level spacing distribution, which probes the short-range correlation, as well as the Σ2 and Δ3 statistics, which are sensitive to the long-range correlations in the spectrum, are investigated and compared to the distributions predicted for regular and irregular spectra. Both of these approaches indicate that the system is almost totally irregular with a Brody parameter of about 92%. In addition, the sum of states at a particular energy, which is extremely important in all statistical models for unimolecular dissociation, is approximately calculated from the volume of classical phase space and found to be in excellent agreement with the exact quantum mechanical result. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A time-dependent (TD) quantum wavepacket calculation of reaction probabilities is reported for the reaction HO + CO → H + CO2 for total angular momentum J=0. The dynamics calculation employs the potential-averaged five-dimensional model (PA5D) and is made possible by using a normalized angular quadrature scheme to minimize the requirement for computer memory. Reaction probabilities are obtained from the ground state as well as rotationally excited state in either one of the reactant diatoms. Strong resonances are found in the present study and calculated reaction probabilities are dominated by many narrow and overlapping resonances. These features are in qualitative agreement with several lower dimensional quantum dynamics studies. However, quantitative comparison of the present result with previously reported quantum calculations, including a recent planar four-dimensional (4D) calculation of Goldfield et al., shows that our calculated reaction probabilities are much smaller than those found in reduced dimensionality calculations. We also found reaction probability to be more sensitive to the rotational motion of CO than of HO. In addition to reaction probabilities, the bound state calculation for the stable intermediate complex HOCO has also been carried out and energies of several low-lying vibrational states are obtained. The potential energy surface (PES) of Schatz–Fitzcharles–Harding (SFH) is used in all the calculates presented in this paper. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The aim of this paper is to explore the possibilities of a novel molecular integral equation approach to produce information on the atom–atom microscopic structure of molecular fluids via multidimensional integration of the molecular pair distribution function. In particular, atom–atom structure factors for hydrogen halides (HCl and HI) are computed from the integral equation for heteronuclear fluids modeled by a two-center Lennard-Jones potential with and without multipole terms. Theoretical results are compared both with experimental partial structure factors and computer simulation results. Theory and simulation agree remarkably well both for thermodynamics and microscopic structure. The comparison with experimental partial structure factors is satisfactory within the limitations due to the rough modeling used for describing the real fluid. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the 12C isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals are reproduced to better than 10 cm-1, except for three cases where the error is 11 cm-1. Our calculated harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: Both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch–bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the ν8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper via the World-Wide Web. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Although it is usually stated that the Hartree–Fock method formally scales as N4, where N is the number of basis functions employed in the calculation, it is also well known that mathematical bounds computed with the Schwarz inequality can be used to screen and eliminate four-center two-electron integrals smaller than a certain threshold. In this work, quantitative data is presented to illustrate the effects of this integral screening on the scaling properties of the Hartree–Fock (HF) method. Calculations are performed on a range of carbon–hydrogen model systems, two-dimensional graphitic sheets, and three-dimensional diamond pieces, to determine the effective scaling exponent α of the computational expense. The data obtained in this paper for calculations including over 250 carbon atoms and 1500 basis functions shows two significant trends: (1) in the asymptotic limit of large molecules, α is found to be approximately 2.2–2.3, and (2) for molecules of modest size, α is still very much less than 4. Therefore, integral screening is quantitatively shown to substantially reduce the Hartree–Fock scaling from its formal value of N4. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We show in this paper a time-dependent (TD) quantum wave packet calculation for the combustion reaction H+O2 using the DMBE IV (double many-body expansion) potential energy surface which has a deep well and supports long-lived resonances. The reaction probabilities from the initial states of H+O2(3Σ-g) (v=0–3, j=1) for total angular momentum J=0 are obtained for scattering energies from threshold up to 2.5 eV, which show numerous resonance features. Our results show that, by carrying out the wave packet propagation to several picoseconds, one can resolve essentially all the resonance features for this reaction. The present TD results are in good agreement with other time-independent calculations. A particular advantage of the time-dependent approach to this reaction is that resonance structures—strong energy dependence of the reaction probability—can be mapped out in a single wave packet propagation without having to repeat scattering calculations for hundreds of energies. We also report calculations of some low-lying vibrational energies of the hydroperoxyl radical HO2(2A‘) and their spectroscopic assignments. The vibrational frequencies of HO2(2A‘) on the DMBE IV potential energy surface are lower than experimental values, indicating the need to further improve the accuracy of the potential energy surface. [ABSTRACT FROM AUTHOR]

This paper presents evidence that both hydrogens of the ethylenic and phenylic groups of trans-stilbene interact to stabilize the planar structure of the molecule. Also, it proposes a new model that the photoisomerization (PI) arises from the cutting off process of these intramolecular hydrogen interactions (IMHI). The photoisomerization dynamics of stilbene has been investigated by semiempirical quantum chemistry calculations. This new model yields good agreement with experimental results and specifies the PI/IMHI mechanism in detail. [ABSTRACT FROM AUTHOR]