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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]

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]

The new state-selective (SS) multireference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) particle-hole formalism, which we have introduced in our recent paper [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)], has been implemented and the results of the pilot calculations for the minimum basis-set (MBS) model composed of eight hydrogen atoms in various geometrical arrangements are presented. This model enables a continuous transition between degenerate and nondegenerate regimes. Comparison is made with the results of SR CC calculations involving double (CCD), single and double (CCSD), single, double, and triple (CCSDT), and single, double, triple, and quadruple (CCSDTQ) excitations. Our SS CC energies are also compared with the results of the Hilbert space, state-universal (SU) MR CC(S)D calculations, as well as with the MR configuration interaction (CI) results (with and without Davidson-type corrections) and the exact correlation energies obtained using the full CI (FCI) method. Along with the ground-state energies, we also analyze the resulting wave functions by examining some selected cluster components. This analysis enables us to assess the quality of the resulting wave functions. Our SS CC theory truncated at double excitations, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ formalism [SS CCSD (TQ) method] provides equally good results in nondegenerate and quasidegenerate regions. The difference between the ground-state energy obtained with the SS CCSD(TQ) approach and the FCI energy does not exceed 1.1 mhartree over all the geometries considered. This value compares favorably with the maximum difference of 2.8 mhartree between the SU CCSD energies and the FCI energies obtained for the same range of geometries. The SS CCSD(T) method, emerging from the SR CCSDT theory through selection of the most essential clusters, is less stable, since it neglects very... [ABSTRACT FROM AUTHOR]

We present the results of a quasiclassical trajectory study of vibration–rotation excitation and reaction in H+H2O(000) and H+D2O(000) collisions, including detailed comparisons with experiment. All calculations have used a semiempirical potential surface due to Schatz and Elgersma, and the H2O initial and final states were numerically determined by solving for the good action variables associated with vibrational motions. Our studies of collisional excitation emphasize comparisons with recent experiments by Lovejoy, Goldfarb, and Leone [J. Chem. Phys. 96, 7180 (1992)] in which fast hydrogen atoms produce vibrationally and rotationally excited water. As in the experiments, we find a propensity for the production of rotational states in which the rotational angular momentum vector is predominantly aligned perpendicular to the water molecule plane (c-axis excitation). This propensity is found for all excited vibrational states of H2O, but it is significantly stronger in the experiments [where only the (001) state was studied] than in the calculations. An analysis of trajectory motions indicates that the primary excitation mechanism for states which show the c-axis propensity involves a nearly planar collision in which the incoming H impulsively strikes one of the water hydrogens. Failed reactive collisions associated with either abstraction or exchange as well as reactive exchange collisions give the same propensity but they are not the dominant mechanism for producing aligned water. In studies of the reaction H+D2O→OD+HD, we analyze product vibrational and rotational state distributions in detail, making comparison with recent studies of Adelman, Filseth, and Zare [preceding paper, J. Chem. Phys. 98, 4636 (1993)] as well as earlier work. The product HD energy partitioning is found to be in excellent average agreement with experiment, with the HD receiving much more of the available energy than does OD. There are, however, differences in some of the... [ABSTRACT FROM AUTHOR]

A new approach to calculating quantum functional sensitivity maps of transition probabilities is described in this paper. This approach is based on the log-derivative version of the Kohn variational principle and is applied here to the collinear H+H2 hydrogen exchange reaction. The sensitivity maps provide detailed quantitative information about how variations in the potential energy surface affect the state-to-state transition probabilities. The key issues investigated are (i) the evolution of sensitivity structure in the 0.30–1.50 eV range of total energy; (ii) the comparison of sensitivity structure on the Porter–Karplus, the Liu–Siegbahn–Truhlar–Horowitz, and the double-many-body-expansion potential energy surfaces; and (iii) the range of linearity for first order sensitivity predictions. [ABSTRACT FROM AUTHOR]

Infrared spectra are reported for thin films of deuterated microporous amorphous ice formed at 12 K and saturated with absorbed molecular hydrogen. This paper focuses on both the influence of the surface-bound H2 on the absorption bands of the OD groups that dangle from the micropore surfaces and the behavior of the induced infrared bands of the stretching mode of H2 itself. Both structural changes and the relaxation of ortho-H2 to para-H2 are apparent from variations in the observed spectra with time and temperature. A reasonably detailed interpretation of the complex spectral behavior has been possible through simulation of spectra for H2 interacting with the surface of amorphous ice clusters generated previously in a classical trajectory study of the cluster growth through the accumulation and relaxation of individual water molecules. Potential minima were calculated with respect to H2 coordinates on the cluster surface and a qualitative interpretation of adsorbate–surface bonding provided via the partitioning of the H2··D2O interactions into relatively weak van der Waals interactions, and stronger electrostatic interactions between the H2 quadrupole and the H2O dipole and quadrupole. The net binding of H2 to the surface is dominated by several van der Waals interactions with neighboring D2O molecules, but in most of the calculated minima, H2 also forms a single electrostatic bond to a surface molecule. While such electrostatic bonds provide only a small fraction of the binding energy, they appear to influence strongly the observed spectra. Observed H2-induced shifts in the dangling OD bands appear to be caused by electrostatic bonding of H2 to a D atom of a dangling OD and a significant fraction of the H2 intensity is proposed to originate from H2 molecules which are electrostatically bonded to dangling oxygen atoms on the surface. Calculations (which do not contain adjustable parameters) reproduce quite well several features of the... [ABSTRACT FROM AUTHOR]

The hydrogen transfer reaction catalysed by soybean lipoxygenase (SLO) has been the focus of intense study following observations of a high kinetic isotope effect (KIE). Today high KIEs are generally thought to indicate departure from classical rate theory and are seen as a strong signature of tunnelling of the transferring particle, hydrogen or one of its isotopes, through the reaction energy barrier. In this paper, we build a qualitative quantum rate model with few free parameters that describes the dynamics of the transferring particle when it is exposed to energetic potentials exerted by the donor and the acceptor. The enzyme's impact on the dynamics is modelled by an additional energetic term, an oscillatory contribution known as "gating." By varying two key parameters, the gating frequency and the mean donor-acceptor separation, the model is able to reproduce well the KIE data for SLO wild-type and a variety of SLO mutants over the experimentally accessible temperature range. While SLO-specific constants have been considered here, it is possible to adapt these for other enzymes.

There are many atomistic simulation methods with very different costs, accuracies, transferabilities, and numbers of empirical parameters. I show how statistical model selection can compare these methods fairly, even when they are very different. These comparisons are also useful for developing new methods that balance cost and accuracy. As an example, I build a semiempirical model for hydrogen clusters. [ABSTRACT FROM AUTHOR]

We present a classical density functional theory (DFT) for fluid mixtures that is based on a third-order thermodynamic perturbation theory of Feynman-Hibbs-corrected Mie potentials. The DFT is developed to study the interfacial properties of hydrogen, helium, neon, deuterium, and their mixtures, i.e., fluids that are strongly influenced by quantum effects at low temperatures. White Bear fundamental measure theory is used for the hard-sphere contribution of the Helmholtz energy functional, and a weighted density approximation is used for the dispersion contribution. For mixtures, a contribution is included to account for non-additivity in the Lorentz–Berthelot combination rule. Predictions of the radial distribution function from DFT are in excellent agreement with results from molecular simulations, both for pure components and mixtures. Above the normal boiling point and 5% below the critical temperature, the DFT yields surface tensions of neon, hydrogen, and deuterium with average deviations from experiments of 7.5%, 4.4%, and 1.8%, respectively. The surface tensions of hydrogen/deuterium, para-hydrogen/helium, deuterium/helium, and hydrogen/neon mixtures are reproduced with a mean absolute error of 5.4%, 8.1%, 1.3%, and 7.5%, respectively. The surface tensions are predicted with an excellent accuracy at temperatures above 20 K. The poor accuracy below 20 K is due to the inability of Feynman–Hibbs-corrected Mie potentials to represent the real fluid behavior at these conditions, motivating the development of new intermolecular potentials. This DFT can be leveraged in the future to study confined fluids and assess the performance of porous materials for hydrogen storage and transport. [ABSTRACT FROM AUTHOR]

An alternative approach to density functional theory based on self-consistent field theory for ring polymers is applied to neutral atoms hydrogen to neon in their ground-states. The spontaneous emergence of an atomic shell structure and spherical symmetry-breaking of the total electron density are predicted by the model using the ideas of polymer excluded-volume between pairs of electrons to enforce the Pauli-exclusion principle and an exact electron self-interaction correction. The Pauli potential is approximated by neglecting inter-atomic correlations along with other types of correlations, and comparisons to Hartree–Fock theory are made, which also ignores correlations. The model shows excellent agreement with Hartree–Fock theory to within the standards of orbital-free density functional theory for the atomic binding energies and density profiles of the first six elements, providing exact matches for the elements hydrogen and helium. The predicted shell structure starts to deviate significantly past the element neon, and spherical symmetry-breaking is first predicted to occur at carbon instead of boron. The self-consistent field theory energy functional that describes the model is decomposed into thermodynamic components to trace the origin of spherical symmetry-breaking. It is found to arise from the electron density approaching closer to the nucleus in non-spherical distributions, which lowers the energy despite resulting in frustration between the quantum kinetic energy, electron–electron interaction, and the Pauli exclusion interaction. The symmetry-breaking effect is found to have a minimal impact on the binding energies, which suggests that the spherical-averaging approximation used in previous work is physically reasonable when investigating atomic systems. The pair density contour plots display behavior similar to polymer macro-phase separation, where individual electron pairs occupy single lobe structures that together form a dumbbell shape analogous to the 2p orbital shape. It is further shown that the predicted densities satisfy known constraints and produce the same total electronic density profile that is predicted by other formulations of quantum mechanics. [ABSTRACT FROM AUTHOR]

Non-Maxwellian evaporation of light atoms and molecules (particles) such as He and H2 from liquids has been observed experimentally. In this work, we use simulations to study systematically the evaporation of Lennard-Jones particles from liquid water. We find instances of sub- and super-Maxwellian evaporation, depending on the mass of the particle and the particle-water interaction strength. The observed trends are in qualitative agreement with experiment. We interpret these trends in terms of the potential of mean force and the effectiveness and frequency of collisions during the evaporation process. The angular distribution of evaporating particles is also analyzed, and it is shown that trends in the energy from velocity components tangential and normal to the liquid surface must be understood separately in order to interpret properly the angular distributions. [ABSTRACT FROM AUTHOR]

We performed fixed-node diffusion Monte Carlo (DMC) calculations to investigate structural and energetic properties of graphenylene (GPNL), a two-dimensional network of sp2-bonded carbon atoms with large near-circular pores, and its H2 separation performance for gas mixtures. We have found that the energetic stability of a GPNL monolayer is comparable to that of γ-graphyne, as evidenced by its large cohesive energy of 6.755(3) eV/atom. Diffusion barriers of several gas molecules, including hydrogen, through a GPNL membrane were determined from the analysis of their adsorption energies depending on the adsorption distance, which led to our estimation for hydrogen selectivity with respect to other target molecules. DMC hydrogen selectivity of a GPNL monolayer was found to be exceptionally high at 300 K, as high as 1010–1011 against CO and N2 gases. This, along with high hydrogen permeance due to its generic pore structure, leads us to conclude that GPNL is a promising membrane to be used as a high-performance hydrogen separator from gas mixtures. We find that when compared to our DMC results, DFT calculations tend to overestimate H2 selectivity, which is mostly due to their inaccurate description of short-range repulsive interactions. [ABSTRACT FROM AUTHOR]

The design of nuclear reactors and neutron moderators require a good representation of the interaction of low energy (E < 1 eV) neutrons with hydrogen and deuterium containing materials. These models are based on the dynamics of the material, represented by its vibrational spectrum. In this paper, we show calculations of the frequency spectrum for light and heavy water at room temperature using two flexible point charge potentials: SPC-MPG and TIP4P/2005f. The results are compared with experimental measurements, with emphasis on inelastic neutron scattering data. Finally, the resulting spectra are applied to calculation of neutron scattering cross sections for these materials, which were found to be a significant improvement over library data. [ABSTRACT FROM AUTHOR]

Within the general axially nonadiabatic channel approach described in Paper I of this series [M. Auzinsh, E. I. Dashevskaya, I. Litvin, E. E. Nikitin, and J. Troe, J. Chem. Phys. 139, 084311 (2013)], the present article analyzes the simultaneous manifestation of electrostatic and gyroscopic interactions in the quantum capture of dipolar polarizable symmetric top molecules by ions. As a demonstration, the rate coefficients for capture of CH3D and CD3H by H(+), D(+), and H3(+) are calculated.

This paper gives a natural orbital (NO) based analysis of the van der Waals interaction in (singlet) H2 at long distance. The van der Waals interaction, even if not leading to a distinct van der Waals well, affects the shape of the interaction potential in the van der Waals distance range of 5-9 bohrs and can be clearly distinguished from chemical bonding effects. In the NO basis the van der Waals interaction can be quantitatively covered with, apart from the ground state configurations (1σ(g))(2) and (1σ(u))(2), just the 4 configurations (2σ(g))(2) and (2σ(u))(2), and (1π(u))(2) and (1π(g))(2). The physics of the dispersion interaction requires and explains the peculiar relatively large positive CI coefficients of the doubly excited electron configurations (2σ(u))(2) and (1π(g))(2) (the occupancy amplitudes of the 2σ(u) and 1π(gx, y) NOs) in the distance range 5-9 bohrs, which have been observed before by Cioslowski and Pernal [Chem. Phys. Lett. 430, 188 (2006)]. We show that such positive occupancy amplitudes do not necessarily lead to the existence of zero occupation numbers at some H-H distances.

Characteristic parameters (energetic positions, widths, etc.) of the 23 spectroscopically observed (or potentially observable) quasi-bound energy levels for the ground state H2 molecule are determined with high accuracy. The recent advances concerning nonadiabatic, relativistic, and quantum electrodynamical corrections have been taken into consideration. Theoretical approach is based on the careful investigation of the energy dependences of the real stationary scattering-state wave functions (their phase shifts and amplitudes). These energy dependences can be very accurately fitted to simple analytic formulas resulting from the quantum scattering theory. General criteria for the assignment of quasi-bound states are discussed and a new criterion is proposed, which excludes too broad and ambiguous resonances. To implement the proposed approach, a special high-precision computational method was used, which is described in full detail. [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]

We perform spin-polarized density functional theory calculations for a hydrogen atom interacting exohedrally with a (5,5) single-walled carbon nanotube (SWNT). We also perform full three-dimensional (3D) quantum dynamics calculations of the H atom bound rovibrational states. We initially focus on the four sites of highest symmetry, along which we compute potential energy surface (PES) values at 33 separate, nonuniformly spaced radial values. These 132 geometries are sufficient to define the primary potential interaction regions. We find a weak physisorptive region between 2.5 and 3.5 Å from the SWNT wall, with a maximum well depth of 51 meV, relative to the desorption limit. We also find a chemisorptive region, extending from about 1.0 out to 1.5 Å from the SWNT wall. The maximum well depth of 0.755 eV occurs at 1.15 Å from the SWNT wall, nearly directly above a carbon atom. A small barrier of 54 meV lies between these two binding regions. There are also two types of transition states that lie between adjacent chemisorption wells. In addition to the high-symmetry sites, a detailed and accurate characterization of the PES requires density functional theory calculations along a large number of interstitial sites—18 in all. Using these 18×33 geometries, and exploiting the full D10h symmetry of the system, we fit a global analytical PES, using a Fourier basis in the cylindrical coordinates, with radially dependent expansion coefficients (rms error 3.8 meV). We then perform a mixed spectral basis/phase-space optimized discrete variable representation calculation of all bound rovibrational H atom eigenfunctions and energy levels. We also discuss ramifications for the possible use of SWNTs as hydrogen storage devices. [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]

The free energy surfaces of a wide variety of systems encountered in physics, chemistry, and biology are characterized by the existence of deep minima separated by numerous barriers. One of the central aims of recent research in computational chemistry and physics has been to determine how transitions occur between deep local minima on rugged free energy landscapes, and transition path sampling (TPS) Monte-Carlo methods have emerged as an effective means for numerical investigation of such transitions. Many of the shortcomings of TPS-like approaches generally stem from their high computational demands. Two new algorithms are presented in this work that improve the efficiency of TPS simulations. The first algorithm uses biased shooting moves to render the sampling of reactive trajectories more efficient. The second algorithm is shown to substantially improve the accuracy of the transition state ensemble by introducing a subset of local transition path simulations in the transition state. The system considered in this work consists of a two-dimensional rough energy surface that is representative of numerous systems encountered in applications. When taken together, these algorithms provide gains in efficiency of over two orders of magnitude when compared to traditional TPS simulations. [ABSTRACT FROM AUTHOR]

The dynamics of the guanine-cytosine base pair has been studied in the time-dependent quantum approach. A six-dimensional model involving the nonlinear three hydrogen bridges has been utilized. The modifications induced in the hydrogen transfer from a base to the other by the explicit inclusion of the out-of-plane hydrogen atom position in the three bridges have been evidenced and the consequences on stacking interaction and base pair opening are considered. The relevance of these aspects in biological properties has been suggested. [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]

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]

A state-selective multiconfigurational single-reference method that was outlined in the preceding paper of this series (H. Meißner and J. Paldus, J. Chem. Phys. 113, 2594 (2000); preceding paper), and is based on a quadratic iterative algorithm enabling the direct solution of the generalized Bloch equation, is applied to several model systems consisting of interacting hydrogen molecules, nowadays referred to as the H4, S4, and H8 models. These exactly solvable models are often used to test the efficacy of post-Hartree-Fock methods in their ability to recover both the dynamic and nondynamic correlation energies, since they enable a continuous variation of the degree of quasidegeneracy from the degenerate to nondegenerate limit by varying a single geometrical parameter, while simulating the dissociation of one or more single bonds. Various approximation schemes that were outlined in Part II, as well as their combinations, are tested and their performance evaluated. The size-extensivity deviations of those approximations that do not rely on the exponential cluster ansatz for the wave operator are also examined using larger hydrogen molecule clusters. It is shown that the so-called BQ[sub 4] approximation performs extremely well in all cases and even outperforms the externally corrected, reduced multireference (RMR) CCSD in the quasidegenerate region of geometries. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We describe a simple scheme to perform phonon calculations with quantum Monte Carlo (QMC) methods and demonstrate it on metallic hydrogen. Because of the energy and length scales of metallic hydrogen and the statistical noise inherent to QMC methods, the conventional manner of calculating force constants is prohibitively expensive. We show that our alternate approach is nearly 100 times more efficient in resolving the force constants needed to calculate the phonon spectrum in the harmonic approximation. This requires only the calculation of atomic forces, as in the conventional approach, and otherwise little or no programmatic modification. [ABSTRACT FROM AUTHOR]

The quantum instanton approximation for thermal rate constants of chemical reactions [Miller, Zhao, Ceotto, and Yang, J. Chem. Phys. 119, 1329 (2003)], which is modeled after the earlier semiclassical instanton approach, is applied to the hydrogen abstraction reaction from methane by a hydrogen atom, H+CH[sub 4]→H[sub 2]+CH[sub 3], using a modified and recalibrated version of the Jordan–Gilbert potential surface. The quantum instanton rate is evaluated using path integral Monte Carlo approaches based on the recently proposed implementation schemes [Yamamoto and Miller, J. Chem. Phys. 120, 3086 (2004)]. The calculations were carried out using the Cartesian coordinates of all the atoms (thus involving 18 degrees of freedom), thereby taking explicit account of rotational effects of the whole system and also allowing the equivalent treatment of the four methane hydrogens. To achieve such a treatment, we present extended forms of the path integral estimators for relevant quantities that may be used for general N-atom systems with any generalized reaction coordinates. The quantum instanton rates thus obtained for the temperature range T=200–2000 K show good agreement with available experimental data, which gives support to the accuracy of the underlying potential surface used. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Four implicit potential energy surfaces (PESs) with specific-reaction-parameters (SRP) are developed and tested for the reaction CH[sub 4]+H→CH[sub 3]+H[sub 2]. The first is called MPW60 and is based on the modified Perdew–Wang hybrid density-functional method with the percentage of the Hartree–Fock exchange equal to 60%. The other three PESs are constructed with multi-coefficient correlation methods (MCCMs). The second is called MCOMP2-SRP, and the third is called MC-QCISD-SRP. Both of them are parametrized for this specific reaction by starting with their corresponding global parameters. The fourth is called MCG3-SRP and is based on the MCG3-CHO semiglobal parametrization with further refinement for this specific reaction. All four SRP surfaces have a classical forward barrier height of 14.8 kcal/mol, and all three MCCM SRP surfaces have a classical endoergicity of 3.3 kcal/mol. The stationary point geometries, vibrational frequencies, and zero-point-energies are reported for several standard single-level methods and MCCMs with global parameters as well as for the four new SRP surfaces. Direct dynamics calculations are carried out using variational transition state theory with multidimensional tunneling contributions on the proposed SRP surfaces. We calculate forward reaction rate constants for the title reaction from 250 to 2400 K and compare them with the latest re-analyzed experimental results over the temperature range from 348 to 1950 K. The calculated rate constants using canonical variational theory with the small-curvature tunneling approximation carried out on the MPW60, MC-QCISD-SRP, and MCG3-SRP surfaces show good agreement with the experimental results. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

For the atoms with Z ≤ 11, energies obtained using the "initiator" extension to full configuration interaction quantum Monte Carlo (i-FCIQMC) come to within statistical errors of the FCIQMC results. As these FCIQMC values have been shown to converge onto FCI results, the i-FCIQMC method allows similar accuracy to be achieved while significantly reducing the scaling with the size of the Slater determinant space. The i-FCIQMC electron affinities of the Z ≤ 11 atoms in the aug-cc-pVXZ basis sets are presented here. In every case, values are obtained to well within chemical accuracy [the mean absolute deviation (MAD) from the relativistically corrected experimental values is 0.41 mE(h)], and significantly improve on coupled cluster with singles, doubles and perturbative triples [CCSD(T)] results. Since the only remaining source of error is basis set incompleteness, we have investigated using CCSD(T)-F12 contributions to correct the i-FCIQMC results. By doing so, much faster convergence with respect to basis set size may be achieved for both the electron affinities and the FCIQMC ionization potentials presented in a previous paper. With this F12 correction, the MAD can be further reduced to 0.13 mE(h) for the electron affinities and 0.31 mE(h) for the ionization potentials.

The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5M H2SO4 solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5M H2SO4 saturated with N2 after HER show several characteristic changes: the current waves for hydrogen adsorption in the region of0.2 < E < 0.35 V and for sulfate adsorption at 0.35 < E < 0.5 V decrease and the current spike at 0.44 V for the phase transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer and propose a mechanism by which this enhances hydrogen evolution. [ABSTRACT FROM AUTHOR]

We introduce hybrid gausslet/Gaussian basis sets, where a standard Gaussian basis is added to a gausslet basis in order to increase accuracy near the nuclei while keeping the spacing of the grid of gausslets relatively large. The Gaussians are orthogonalized to the gausslets, which are already orthonormal, and approximations are introduced to maintain the diagonal property of the two electron part of the Hamiltonian so that it continues to scale as the second power of the number of basis functions rather than the fourth. We introduce several corrections to the Hamiltonian designed to enforce certain exact properties, such as the values of certain two-electron integrals. We also introduce a simple universal energy correction that compensates for the incompleteness of the basis stemming from the electron–electron cusps based on the measured double occupancy of each basis function. We perform a number of Hartree Fock and full configuration interaction (full-CI) test calculations on two electron systems and Hartree Fock on a ten-atom hydrogen chain to benchmark these techniques. The inclusion of the cusp correction allows us to obtain complete basis set full-CI results for the two electron cases at the level of several micro-Hartrees, and we see similar apparent accuracy for Hartree Fock on the ten-atom hydrogen chain. [ABSTRACT FROM AUTHOR]

In this work, we present an in-depth study of the role of mass transport conditions in tuning the hydrogen evolution kinetics on gold by means of rotation rate control. Interestingly, we find that the hydrogen evolution reaction (HER) activity decreases with the increasing rotation rate of the electrode. As we increase the rotation (mass transport) rate, the locally generated hydroxyl ions (2H2O +2e− → H2 + 2OH−) are transported away from the electrode surface at an accelerated rate. This results in decreasing local pH and, because of the need to satisfy local electroneutrality, decreasing near-surface cation concentration. This decrease in the near-surface cation concentration results in the suppression of HER. This is because the cations near the surface play a central role in stabilizing the transition state for the rate determining Volmer step (*H–OHδ−–cat+). Furthermore, we present a detailed analytical model that qualitatively captures the observed mass transport dependence of HER solely based on the principle of electroneutrality. Finally, we also correlate the cation identity dependence of HER on gold (Li+ < Na+ < K+) to the changes in the effective concentration of the cations in the double layer with the changes in their solvation energy. [ABSTRACT FROM AUTHOR]

Accurate calculation of the energies and widths of the resonances of HOCl--an important intermediate in the O(1D)HCl reactive system--poses a challenging benchmark for computational methods. The need for very large direct product basis sets, combined with an extremely high density of states, results in difficult convergence for iterative methods. A recent calculation of the highly excited OH stretch mode resonances using the filter diagonalization method, for example, required 462,000 basis functions, and 180,000 iterations. In contrast, using a combination of new methods, we are able to compute the same resonance states to higher accuracy with a basis less than half the size, using only a few hundred iterations-although the CPU cost per iteration is substantially greater. Similar performance enhancements are observed for calculations of the high-lying bound states, as reported in a previous paper [J. Theo. Comput. Chem. 2, 583 (2003)]., (Copyright 2004 American Institute of Physics)

Studies dissociation processes in three-dimensional helium+hydrogen collisions using a semiclassical approach which treats the relative radial motion in classical mechanics and other motions in quantum mechanics. Centrifugal sudden approximation assumed to solve the semiclassical equation; Comparison of results with previous full quantum mechanical calculations.

The catalytic reduction in carbon dioxide is a crucial step in many chemical industrial reactions, such as methanol synthesis, the reverse water–gas shift reaction, and formic acid synthesis. Here, we investigate the role of bulk hydrogen, where hydrogen atoms are found deep inside a metal surface as opposed to subsurface ones, upon CO2 reduction over a Ni(110) surface using density functional theory and ab initio molecular dynamics simulations. While it has previously been shown that subsurface hydrogen stabilizes CO2 and can aid in overcoming reaction barriers, the role of bulk hydrogen is less studied and thus unknown with regard to CO2 reduction. We find that the presence of bulk hydrogen can significantly alter the electronic structure of the Ni(110) surface, particularly the work function and d-band center, such that CO2 adsorbs more strongly to the surface and is more easily reduced. Our results show an enhanced CO2 dissociation in the presence of bulk hydrogen, shedding light on a hitherto underappreciated mechanistic pathway for CO2 reduction on metal surfaces. [ABSTRACT FROM AUTHOR]

This paper reports the first product differential cross section (DCS) measurements for the D+H2→DH+H reaction as a function of laboratory (LAB) scattering angle with sufficient resolution to resolve product DH vibrational states. Using a D-atom beam produced by the photodissociation of DI at 248 nm, product velocity and angular distributions were measured at 12 LAB angles at a nominal collision energy of 0.53 eV and at 22 LAB angles at a nominal collision energy of 1.01 eV with a crossed molecular beams apparatus. After correction of the raw product time-of-flight (TOF) spectra for modulated background, a comparison with recent exact quantum mechanical scattering calculations was made using a Monte Carlo simulation of the experimental conditions. The simulation showed that although the theoretical predictions agree qualitatively with the measurements, some significant discrepancies exist. Using the Monte Carlo simulation, a best-fit set of DH(v,j) DCS’s which showed good agreement with the measurements was found. At the detailed level of the state-to-state DCS, significant differences were observed between theory and experiment for rotationally excited DH(v, j) products. The discrepancies observed suggest that some regions of the current ab initio H3 potential energy surfaces, particularly the bending potential at high energies, may need further examination. [ABSTRACT FROM AUTHOR]

The 5g–4 f Rydberg groups of H2 and D2 first studied in paper I have been obtained with a tenfold increase in resolution which made it possible to resolve the singlet from the triplet components. As a result we can now establish separately precise values for the energy levels in the triplet and singlet systems. For this purpose we have remeasured a number of transitions between the lower energy levels for which at present only old measurements are available. In particular we obtain accurate values for the energies of the lowest (stable) triplet state a3Σ+g relative to the singlet ground state, as well as of the ionization potential. The values obtained for the former are more accurate than obtained from singlet–triplet anticrossings while the latter are of similar accuracy as those reported recently by McCormack et al. [Phys. Rev. A 39, 2260 (1989)] and fit well within this accuracy with the most recent ab initio values. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed study of dissociation and recombination of H2 over the temperature range 1000 to 5000 K. The chemical processes are modeled by solving the master equation for the concentrations of the full set of rovibration states. All of the state-to-state energy transfer rate coefficients required by the master equation are evaluated by the quasiclassical trajectory method using a potential energy surface which is a fit to ab initio electronic structure calculations. The analysis of the results of the master equation to obtain the phenomenological rate coefficients was carried out using several techniques. This is required because there is a mixture of third bodies and their concentrations change as the reaction proceeds. The methods based on one-way fluxes are not reliable, while the methods based upon an eigenvalue solution of the linearized master equation are in reasonable agreement with our preferred method, which is based on fitting the concentrations from a two component master equation. The results from various methods of determining the phenomenological rate coefficient for H2 as a third body are in good agreement with experimental estimates. The recombination rate coefficients are most sensitive to the collision induced dissociation rates of initial states with moderate values of vibrational and rotational quantum numbers. A comparison of the orbiting resonance theory to the accurate results shows that this simple model is not valid under the conditions of the present study. [ABSTRACT FROM AUTHOR]

This paper examines the effect of infinitesimal functional variations in a rigid rotor He–H2 potential surface on several different levels of observables: inelastic cross sections, rate constants, and energy level populations. Equations are derived for the functional derivatives of these observables with respect to a variation in the potential surface. Sensitivities are presented with respect to the entire potential surface, as well as the individual Legendre components Vn(r). The dynamical and kinetic observables studied were found to be most sensitive to the V2(r) term in the potential with the region of highest sensitivity dependent upon the energy or temperature as well as the states related by the individual observable. Sensitivity to the entire surface tends to show more structure due to interference among sensitivities to the individual components. While the main information on the underlying potential is retained, some information loss has been observed in the transition from the microscopic observables to the macroscopic ones. [ABSTRACT FROM AUTHOR]

This paper presents the first ab initio attempt to reconstruct the observed band profile of the stretching fundamental vFX (X=H,D) in the FX···O(CH3)2 hydrogen-bonded system. The two-dimensional potential energy surface V(rFH,RF···O) is evaluated by means of large basis set SCF calculations. The related force constants up to the fourth order are obtained via the analytical fit to a polynomial expansion. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The side bands of the stretching fundamental vFX are described in terms of the vFX ±nvFX···O combination bands in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

We introduce a spin dependent force model for molecular liquids, in which the force between two atoms depends both on the interatomic separation distance and internal, quantum degrees of freedom. In this paper we introduce an additional approximation and treat the internal degrees of freedom semiclassically. We explore some simple properties of this model applied to hydrogen atoms and molecules. Using input from quantum chemistry calculations for the electronic potential energy surfaces, we calculate an effective molecule–molecule potential energy and the dynamics of two and three atom systems. We also perform molecular dynamic simulations which propagate both position and internal ‘‘spin’’ degrees of freedom. [ABSTRACT FROM AUTHOR]

This paper explores the usefulness of the time-dependent self-consistent field (TDSCF) approximation for treating the dynamics of a reaction coordinate coupled to a bath of harmonic degrees of freedom. The reaction coordinate is a one-dimensional double well potential, typical of a hydrogen atom isomerization process. The standard (i.e., one configuration) TDSCF approximation is found to provide a very poor description of the effect of coupling to the bath on the isomerization rate. A multiconfiguration (MC-TDSCF) treatment is thus developed and found to provide a much improved description. [ABSTRACT FROM AUTHOR]

This paper presents the detailed results of a quasiclassical trajectory surface hopping study of reaction of highly vibrationally excited H+2 with ground state H2 (and isotopic counterparts H+2 +D2, D+2 +H2, and D+2 +D2 ), with particular emphasis on the formation and decay of metastable H+3 products. A diatomics-in-molecules surface is used which has been successful in previous studies of H+2 (v) + H2 at low v. In the present study, we consider v=0–17, and find that metastable H+3 ’s are a major product for v≥13. Some of these metastables decay rapidly, showing exponential lifetime distributions with 2–7 ps lifetimes depending on v and on isotope. The remaining H+3 ’s have much longer lifetimes, and a number of methods are used to determine the origin of their stability. In no cases are any of these molecules found to be quasiperiodic, but a Fourier spectral analysis reveals some decoupling of H+ –H2 orbital motion from H2 rotational motion, and we find that many molecules have long lifetimes even though they have energies which are above any rigorous centrifugal barrier. The relation of these results to recent infrared absorption measurements on highly excited H+3 ’s is discussed. [ABSTRACT FROM AUTHOR]

The reaction Li+HF→LiF+H is studied using three-dimensional quantum reactive scattering theory. The rotational and bending motions are treated using the bending corrected rotating nonlinear model (BCRNM) developed in the body of this paper. Reaction probabilities and total reaction cross sections are calculated for energies up to 1 eV. Reaction cross sections are compared with experimental results at 0.38 and 0.63 eV. Results are in excellent agreement with experiment at the lower energy. We calculate a total reaction cross section σ of 0.72 Å2 vs the measured value of 0.80 Å2. At the higher energy, the BCRNM overestimates the cross section: We calculate σ=2.46 Å2, while the experimental value is σ=0.97 Å2. [ABSTRACT FROM AUTHOR]