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The variational approach detailed in the previous paper (Paper I) for the treatment of the Renner–Teller effect in linear tetra-atomic molecules including all degrees of freedom and couplings between angular momenta is applied for HCCH+. The accurate six-dimensional potential energy surfaces of the X 2Πu electronic state, presented in Paper I is incorporated in the variational treatment in order to obtain all rovibronic levels including the spin-orbit coupling for 1/2≤J≤7/2 and up to 2600 cm-1 above the global zero point energy. The “pure” stretching levels are calculated up to 11 000 cm-1 from the stretching zero point energy. The calculated rovibronic energies are compared with previous theoretical and experimental data. The mean agreement with the zero kinetic energy photoelectron measurements of Tang et al. [J. Chem. Phys. 125, 133201 (2006)] is of 16.7 cm-1. The Renner–Teller parameters have been determined at νtrans=690.0 cm-1, εtrans=0.30, νcis=715.0 cm-1, and εcis=-0.063. A detailed analysis of the rovibronic Hund’s cases is presented and the rotational structures of some vibronic bands recorded by Yang and Mo [J. Phys. Chem. A 110, 11001 (2006)] are given. [ABSTRACT FROM AUTHOR]

In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron–ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1 a 1 − 1 4 a 1 core-excited molecule in the production of fragment ions. OH+ fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H+ fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1 a 1 − 1 ) to the lower-energy dication states show that the formation of the OH+ + H+ ion pair dominates, whereas sequential fragmentation OH+ + H+ → O + H+ + H+ is observed for transitions to higher dication states, supporting previous theoretical investigations. [ABSTRACT FROM AUTHOR]

In view of the initial success of the complete active space (CAS) based size-extensive state-specific multireference perturbation theory (SS-MRPT) [J. Phys. Chem. A 103, 1822 (1999)] for relatively diverse yet simple chemically interesting systems, in this paper, we present the computation of the potential energy curves (PEC) of systems with arbitrary complexity and generality such as HF, H2O, H2S, C2, and N2 molecules. The ground states of such systems (and also low-lying singlet excited states of C2) possess multireference character making the description of the state difficult with single-reference (SR) methods. In this paper, we have considered the Mo\ller–Plesset (MP) partitioning scheme [SS-MRPT(MP)] method. The accuracy of energies generated via SS-MRPT(MP) method is tested through comparison with other available results. Comparison with FCI has also been provided wherever available. The accuracy of this method is also demonstrated through the calculations of NPE (nonparallelism error) and the computation of the spectroscopic constants of all the above mentioned systems. The quality of the computed spectroscopic constants is established through comparison with the corresponding experimental and FCI results. Our numerical investigations demonstrate that the SS-MRPT(MP) approach provides a balanced treatment of dynamical and non-dynamical correlations across the entire PECs of the systems considered. [ABSTRACT FROM AUTHOR]

We report a series of full close-coupling calculations of integral cross sections for fine structure resolved, rotationally inelastic transitions of CH induced by collisions with He. These calculations use the necessary two CH(X 2Π)+He potential energy surfaces as determined by a variety of ab initio techniques described in the preceeding paper. The calculated N=1→N=2-7 cross sections confirm a previous prediction of preferential population of final state levels in which the electronic wave function of the CH molecule is antisymmetric with respect to reflection in the plane of rotation of the molecule. A generally good overall agreement is found between the energy-dependent cross sections determined in earlier experiments of Macdonald and Liu and appropriate averages of the calculated cross sections. However, small, systematic, qualitative discrepancies persist between theory and experiment. Diagnostic calculations were carried out to identify the cause of these discrepancies. These calculations explored the influence of restrictions in basis set, configuration interaction, and functional forms used in fitting the potential energy surfaces. They also explored the consequences of the dynamical approximations of CH as a rigid rotor and the independence of the CH spin–orbit constant on the approach of the He partner. All these diagnostic calculations generally confirm the anticipated marginal influence of these approximations. [ABSTRACT FROM AUTHOR]

In this paper we extend our previous study (F. Schneider, F. Di Giacomo, and F. A. Gianturco, J. Chem. Phys. 104, 5153) on the topology of the electronic states of the neutral H2O molecule in C2v symmetry by examining the lowest ten potential energy surfaces of the water molecular cation in its doublet states. The relevant electronic energy surfaces of H2O+ are shown as 2D contour maps where possible reaction pathways for several low-lying potential energy surfaces of H2O+ are clearly seen and therefore can be discussed and analyzed in some detail. The present results were obtained using ab initio multireference configuration interaction calculations at 184 nuclear arrangements, as described in our previous paper dealing with the neutral H2O. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The variational approach detailed in the previous paper (Paper I) for the treatment of the Renner–Teller effect in linear tetra-atomic molecules including all degrees of freedom and couplings between angular momenta is applied for HCCH+. The accurate six-dimensional potential energy surfaces of the X 2Πu electronic state, presented in Paper I is incorporated in the variational treatment in order to obtain all rovibronic levels including the spin-orbit coupling for 1/2≤J≤7/2 and up to 2600 cm-1 above the global zero point energy. The “pure” stretching levels are calculated up to 11 000 cm-1 from the stretching zero point energy. The calculated rovibronic energies are compared with previous theoretical and experimental data. The mean agreement with the zero kinetic energy photoelectron measurements of Tang et al. [J. Chem. Phys. 125, 133201 (2006)] is of 16.7 cm-1. The Renner–Teller parameters have been determined at νtrans=690.0 cm-1, εtrans=0.30, νcis=715.0 cm-1, and εcis=-0.063. A detailed analysis of the rovibronic Hund’s cases is presented and the rotational structures of some vibronic bands recorded by Yang and Mo [J. Phys. Chem. A 110, 11001 (2006)] are given. [ABSTRACT FROM AUTHOR]

This is the second paper in a series of investigations on spin-orbit coupling (SOC) effects in dihydrides of third-row transition elements. The dissociation path of rhenium dihydride was explored using the multiconfiguration self-consistent-field method followed by diagonalization of SOC matrices, in which the Stevens–Basch–Krauss–Jasien–Cundari (SBKJC) basis sets were employed after adding one set of polarization functions for each atom. The most stable rhenium dihydride has a linear structure and its ground state is 6Σg+. Both C2v and Cs dissociation paths into a Re atom and a hydrogen molecule (Re(6S)+H2(1Σg+)) were explored on the potential energy curves of low-lying states. A relatively high energy barrier was obtained along the C2v path and two conical intersections were found at the H–Re–H angles of 29.8° and 96.1° along the C2v path. Since it was revealed that the geometrical deformation to Cs symmetry at the H–Re–H angle of 29.8° does not provide explicit lowering of the energy barrier for the dissociation, even after considering nonadiabatic couplings (NACs) in the neighborhood of the conical intersections, it can be concluded that the most feasible path is hopping from the lowest 6A1 state to the lowest 6B2 state at the H–Re–H angle of 96.1° followed by hopping from the lowest 6B2 state back to the lowest 6A1 state at the H–Re–H angle of 29.8°, where the latter crossing point is the highest in energy along this path. Thus, when the molecular system can reach the areas of these crossing points, the molecular system hops from one of the states to another owing to NAC or SOC effects; especially, SOC effects become important at the crossing point with C2v symmetry. [ABSTRACT FROM AUTHOR]

In this paper, we have computed the rovibrational spectrum of the H3+ molecule using a new global potential energy surface, invariant under all permutations of the nuclei, that includes the long range electrostatic interactions analytically. The energy levels are obtained by a variational calculation using hyperspherical coordinates. From the comparison with available experimental results for low lying levels, we conclude that our accuracy is of the order of 0.1 cm-1 for states localized in the vicinity of equilateral triangular configurations of the nuclei, and changes to the order of 1 cm-1 when the system is distorted away from equilateral configurations. Full rovibrational spectra up to the H++H2 dissociation energy limit have been computed. The statistical properties of this spectrum (nearest neighbor distribution and spectral rigidity) show the quantum signature of classical chaos and are consistent with random matrix theory. On the other hand, the correlation function, even when convoluted with a smoothing function, exhibits oscillations which are not described by random matrix theory. We discuss a possible similarity between these oscillations and the ones observed experimentally. [ABSTRACT FROM AUTHOR]

This paper compares harmonic and anharmonic zero-point energies and thermodynamic functions for a number of molecules of small and medium size. Anharmonic corrections cannot be neglected for quantitative studies, but can be obtained quite effectively by a perturbative treatment including cubic force constants to the second order and semidiagonal quartic constants to the first order. Simple finite difference equations provide all the necessary terms by at most 6N-11 Hessian evaluations, where N is the number of atoms in the system. Accurate values are obtained by this method using the Becke three parameter Lee–Yang–Parr functional, medium size basis sets, and, when needed, proper treatment of internal rotations. The whole model has been completely automated in the Gaussian package. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The goal of this paper is to rationalize the fluorescence activity, experimentally observed for 21 molecules of the same family as the 4-(N,N-dimethyl)anilines, based on their potential energy surfaces calculated within the twisting intramolecular charge transfer model. A classification in four groups is proposed according to the sign of two parameters, ΔE[sup gap], characterizing the energy difference between the vertical locally and charge transfer excited states, and ΔE[sup 1S] (or ΔE[sup 1S] for pretwisted systems), representing the energy gain of the charge transfer excited state with a perpendicular conformation compared to the first vertical excited state. In this study, the time-dependent density-functional theory has been used to calculate the potential energy surfaces of the ground and excited states along the twisting angle. Computed excitation energies and optimized ground state geometries have been obtained with both B3LYP and MPW1PW91 functionals using a 6-311+G(2d,p), and a 6-31G(d) basis set, respectively. From this study, it follows that ΔE[sup gap] and ΔE[sup 1S] are the main parameters necessary to understand the fluorescence activity of these molecules. The fact that the same fluorescence activity is observed for the members of each group (or subcategory for the particular case of group II), reveals the underlying twisting mechanism as a common process for all the investigated molecules, which explains their experimental dual and nondual emission. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Shapes of the potential energy surfaces (PES) of ‘‘floppy’’ molecules can be represented by graphs in which vertices denote global minima and edges connecting the vertices describe energetically accessible paths connecting such global minima. Such graphical representations of PESs are characterized by (i) the number of vertices; (ii) edges that connect (or do not) various vertices; and (iii) the symmetry of the PES, which may be obtained from the graphical figure. These characteristics are shown in this paper to be reflected in the vibration–rotation wave function’s symmetry. Such a symmetry analysis reduces to that provided by the conventional point group for rigid molecules, but provides a useful tool for characterizing vibrations of floppy molecules. In particular, it is shown that for so-called connected graphs, the nonrigid group of the molecule is its full permutation-inversion group. PESs with connected graphs having three and six global minima are examined as examples, and relevant character tables are provided. PESs with disconnected graphs (which characterize species containing some inaccessible barriers), are also considered. The flexible molecules ArH3+, C2H3+, CH4+, and LiBH4 are considered as examples of the tools presented here. [ABSTRACT FROM AUTHOR]

This paper discusses the calculation of the potential energy surfaces (PES) for the electronic singlet states of C2v and C∞v symmetries of a nitrogen molecule interacting with the atomic hydrogen negative ion. The behavior of such surfaces is analyzed as a function of relative orientations and also of the molecular internal coordinate. The PES’s have been obtained using an ab initio, multireference configuration interaction method (MRDCI) and the effects of correlation forces and of basis set size are analyzed in order to understand the role of electron transfer (ET) processes which are likely to take place during closer collisions between partners and which are suggested to be responsible for the vibronic coupling effects which occur during low-energy scattering. The general features of the orientational anisotropy of this interaction, of its dependence on the molecular coordinate, and of the strength of its coupling with the impinging negative ion are also analyzed and discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The δ-target method proposed recently [Chem. Phys. Lett. 235, 309 (1995)] was shown to significantly simplify the task of optimal control of molecular localization. The present paper is intended to study the application of the δ-target technique to a practically interesting case of a diatomic molecule including rotation. Illustrative numerical examples are presented for the molecules NO and Na2. The δ-target method is shown to provide good control when the rotational temperature is not too high. The optimal control solution is very robust and insensitive to small changes in the potential curves and transition dipole moment functions. It is shown that the field obtained within the linearized δ-target optimal control technique can be successfully used in the nonlinear regime. The possibility of extending the δ-target technique is considered for treating strong field nonlinear optimal control tasks. The computational advantages of the method are discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

There has been great progress in developing methods for machine-learned potential energy surfaces. There have also been important assessments of these methods by comparing so-called learning curves on datasets of electronic energies and forces, notably the MD17 database. The dataset for each molecule in this database generally consists of tens of thousands of energies and forces obtained from DFT direct dynamics at 500 K. We contrast the datasets from this database for three "small" molecules, ethanol, malonaldehyde, and glycine, with datasets we have generated with specific targets for the potential energy surfaces (PESs) in mind: a rigorous calculation of the zero-point energy and wavefunction, the tunneling splitting in malonaldehyde, and, in the case of glycine, a description of all eight low-lying conformers. We found that the MD17 datasets are too limited for these targets. We also examine recent datasets for several PESs that describe small-molecule but complex chemical reactions. Finally, we introduce a new database, "QM-22," which contains datasets of molecules ranging from 4 to 15 atoms that extend to high energies and a large span of configurations. [ABSTRACT FROM AUTHOR]

Multireference configuration interaction calculations with the cc-pVQZ basis set are reported for the potential-energy surfaces of the &Xtilde;[SUP2] Ã[SUP2] A[SUB1] &Btilde;[SUP2]B[SUB2] 1[SUP4] A[SUB2] and 1[SUP4] B[SUB1] states of H[SUB2]S[SUP+]. Seams of intersection between the states have been characterized and the conical intersection between the Ã[SUP2] A[SUB1] and &Btilde;[SUP2] B[SUB2] states has been located. Optimized geometries and vertical and adiabatic ionization energies are in good agreement with the available experimental data. The &Xtilde;[SUP2] B[SUB1] state is strongly bound with an equilibrium geometry very similar to that of ground-state H[SUB2]S. The equilibrium bond angle for the Ã[SUP2] A[SUB1] state is 126.4°. The dissociation of this state to S[SUP+]([SUP4]S) + H[SUB2] is interpreted in terms of an initial increase in bond angle on formation and a transition to the &Xtilde;[SUP2] B[SUB1] surface at linearity. As the bond angle decreases the intersection with the 1[SUP4] A[SUB2] state is reached and a nonadiabatic transition to this state results in dissociation. It is suggested that the dissociation of the &Btilde;[SUP2] B[SUB2] state to SH[SUP+]([SUP3]∑[SUP-]) + H occurs via asymmetric stretching followed by a nonadiabatic transition to the dissociative 1[SUP4] A'' surface. [ABSTRACT FROM AUTHOR]

A trajectory surface-hopping study of collisions of H+2 (v)+He for v=0, 3, 6, and 10 has been carried out on the two lowest potential-energy surfaces at relative collision energies of 3.1, 5, and 10 eV. The diatomics-in-molecules (DIM) surfaces of Whitton and Kuntz, suitably modified at large internuclear distances, were used in the calculations. The probability for hopping between the two surfaces was calculated using the Demkov formalism. Both total cross sections and velocity vector distributions are reported. The reactive cross sections to give HeH++H were not affected by the accessibility of the excited potential surface. By comparison, the results for collision-induced dissociation (CID) to give He+H++H were quite revealing. In an earlier paper we have shown that it is possible to distinguish the two (nearly) degenerate product states in CID. The present work shows that between 33% and 45% of the CID products appear in the excited electronic state. The H+ velocity distributions are quite different in the two product states. The results obtained here are compared with other theoretical and experimental work on this system. [ABSTRACT FROM AUTHOR]

This paper reports (a) improved values for low-lying vibration intervals of H+3, H2D+, D2H+, and D+3 calculated using the variational method and Simons–Parr–Finlan representations of the Carney–Porter and Dykstra–Swope ab initio H+3 potential energy surfaces, (b) quartic normal coordinate force fields for isotopic H+3 molecules, (c) comparisons of variational and second-order perturbation theory, and (d) convergence properties of the Lai–Hagstrom internal coordinate vibrational Hamiltonian. Standard deviations between experimental and ab initio fundamental vibration intervals of H+3, H2D+, D2H+, and D+3 for these potential surfaces are 6.9 (Carney–Porter) and 1.2 cm-1 (Dykstra–Swope). The standard deviations between perturbation theory and exact variational fundamentals are 5 and 10 cm-1 for the respective surfaces. The internal coordinate Hamiltonian is found to be less efficient than the previously employed ‘‘t’’ coordinate Hamiltonian for these molecules, except in the case of H2D+. [ABSTRACT FROM AUTHOR]

Reactive cross sections have been computed for the five alkali metals reacting with bromine molecules. The computations were carried out on the potential energy surfaces described in the companion paper. A new type of reaction mechanism, vibrational capture, is predicted to occur for these systems. The calculated cross sections agree well with the experimental data at low collision energies. At higher energies there are indications that not all reactions at small impact parameters are reactive. [ABSTRACT FROM AUTHOR]

The rovibrational energy levels and intensities of the CH3F–H2 dimer have been obtained using our recent global intermolecular potential energy surface [X.-L. Zhang et al., J. Chem. Phys. 148, 124302 (2018)]. The Hamiltonian, basis set, and matrix elements are derived and given for a symmetric top-linear molecule complex. This approach to the generation of energy levels and wavefunctions can readily be utilized for studying the rovibrational spectra of other van der Waals complexes composed of a symmetric top molecule and a linear molecule, and may readily be extended to other complexes of nonlinear molecules and linear molecules. To confirm our method, the rovibrational levels of the H2O–H2 dimer have been computed and shown to be in good agreement with experiment and with previous theoretical results. The rovibrational Schrödinger equation has been solved using a Lanczos algorithm together with an uncoupled product basis set. As expected, dimers containing ortho-H2 are more strongly bound than dimers containing para-H2. Energies and wavefunctions of the discrete rovibrational levels of CH3F–paraH2 complexes obtained from the direct vibrationally averaged 5-dimensional potentials are in good agreement with the results of the reduced 3-dimensional adiabatic-hindered-rotor (AHR) approximation. Accurate calculations of the transition line strengths for the orthoCH3F–paraH2 complex are also carried out, and are consistent with results obtained using the AHR approximation. The microwave spectrum associated with the orthoCH3F–orthoH2 dimer has been predicted for the first time. [ABSTRACT FROM AUTHOR]

We present high-level, coupled-mode calculations of the infrared spectrum of the cyclic formic acid dimer. The calculations make use of full-dimensional, ab initio potential energy and dipole moment surfaces. The potential is a linear least-squares fit to 13 475 CCSD(T)-F12a/haTZ (haTZ means aug-cc-pVTZ basis set for O and C, and cc-pVTZ for H) energies, and the dipole moment surface is a fit to the dipole components, calculated at the MP2/haTZ level of theory. The variables of both fits are all (45) internuclear distances (actually Morse variables). The potential, which is fully permutationally invariant, is the one published recently and the dipole moment surface is newly reported here. Details of the fits, especially the dipole moment, and the database of configurations are given. The infrared spectrum of the dimer is calculated by solving the nuclear Schrödinger equation using a vibrational self-consistent field and virtual-state configuration interaction method, with subsets of the 24 normal modes, up to 15 modes. The calculations indicate strong mode-coupling in the C—H and O—H stretching region of the spectrum. Comparisons are made with experiments and the complexity of the experimental spectrum in the C—H and O—H stretching region is successfully reproduced. [ABSTRACT FROM AUTHOR]

Corrects errors that appeared in the article, entitled 'Energy switching approach to potential surfaces: An accurate single-valued function for the water molecule,' published in the 1996 issue of the 'Journal of Chemical Physics.'

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H2O complexes (D0(para) = 31.67 cm-1, D0(ortho) = 34.66 cm-1) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-> ← |00+> and |02+> ← |00+>) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ ← Σ, Π ← Σ and Σ ← Π infrared bands in the overtone spectra, where Σ (K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne-H2O internuclear axis. End-over-end tumbling of the ortho Ne-H2O cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02+>fΠ(111) ← eΣ(000) para Ne-H2O spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (Trot ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-H2O clusters and provide support for similar cooling dynamics in rare gas-tagging studies. [ABSTRACT FROM AUTHOR]

We present quantum close-coupling calculations for the rotational excitation of the interstellar amidogen radical NH2 due to collisions with H2 molecules. The calculations are based on a recent, high-accuracy full-dimensional NH4 potential energy surface adapted for rigid-rotor scattering calculations. The collisional cross section calculations are performed for all transitions among the first 15 energy levels of both ortho- and para-NH2 and for total energies up to 1500 cm-1. Both paraand ortho-H2 colliding partners are considered. The cross sections for collision with para- and ortho- H2 are found to differ significantly, the magnitude of the ortho-H2 ones being dominant. No strong propensity rules are observed but transitions with Δkc = 0 are slightly favored. [ABSTRACT FROM AUTHOR]

A new finding of the site-averaging approximation was recently reported on the dissociative chemisorption of the HCl/DCl+Au(111) surface reaction [T. Liu, B. Fu, and D. H. Zhang, J. Chem. Phys. 139, 184705 (2013); J. Chem. Phys. 140, 144701 (2014)]. Here, in order to investigate the dependence of new site-averaging approximation on the initial vibrational state of H2 as well as the PES for the dissociative chemisorption of H2 on Cu(111) surface at normal incidence, we carried out six-dimensional quantum dynamics calculations using the initial state-selected time-dependent wave packet approach, with H2 initially in its ground vibrational state and the first vibrational excited state. The corresponding four-dimensional site-specific dissociation probabilities are also calculated with H2 fixed at bridge, center, and top sites. These calculations are all performed based on two different potential energy surfaces (PESs). It is found that the site-averaging dissociation probability over 15 fixed sites obtained from four-dimensional quantum dynamics calculations can accurately reproduce the six-dimensional dissociation probability for H2 (v = 0) and (v = 1) on the two PESs. [ABSTRACT FROM AUTHOR]

An explicit formulation of the rotational relaxation time in terms of state-to-state rate coefficients associated to inelastic collisions is reported. The state-to-state rates needed for the detailed interpretation of relaxation in H2 and D2, including isotopic variant mixtures, have been calculated by solving the close-coupling Schrödinger equations using the H2–H2 potential energy surface by Diep and Johnson [J. Chem. Phys.112, 4465 (2000)]. Relaxation related quantities (rotational effective cross section, bulk viscosity, relaxation time, and collision number) calculated from first principles agree reasonably well with acoustic absorption experimental data on H2 and D2 between 30 and 293 K. This result confirms at once the proposed formulation, and the validation of the H2–H2 potential energy surface employed, since no approximations have been introduced in the dynamics. Accordingly, the state-to-state rates derived from Diep and Johnson potential energy surface appear to be overestimated by up to 10% for H2, and up to 30% for D2 at T = 300 K, showing a better agreement at lower temperatures. [ABSTRACT FROM AUTHOR]

While the use of Frenkel-type models for semiconducting polymer assemblies and related molecular aggregates is well established, the direct parametrization of such models based on electronic structure data is attempted less frequently. In this work, we develop a systematic mapping procedure which is adapted to J-type and H-type homo-aggregate systems. The procedure is based upon the analytic solution of an inverse eigenvalue problem for an effective Frenkel Hamiltonian with nearest-neighbor couplings. Vibronic interactions are included for both site-local and site-correlated modes. For illustration, an application is presented to the excited-state ab initio potential energy surfaces (PESs) of an oligothiophene octamer. The procedure performs a pointwise mapping of the PESs of oligomers of arbitrary chain length n, provided that the electronic ground state and any two of the n lowest adiabatic states of the excitonic manifold of interest are known. These three states are reproduced exactly by the procedure while the remaining n - 2 states of the excitonic manifold can be predicted. Explicit conditions are derived permitting to verify whether a given data set is compatible with the effective Frenkel model under study. [ABSTRACT FROM AUTHOR]

The nitric oxide (NO) is a notorious compound for polluting environment. Recent year, removing nitric oxide from the atmosphere becomes a focus of the investigation. In our work, we study the iminovinylidene (HNCC) radical reacted with NO molecule. The mechanism and kinetic for reaction of the HNCC radical with the NO molecule is investigated via considering the possible channels of the N and O atoms of NO attacking the N and C atoms of the HNCC based on the high level ab initio molecular orbital calculations in conjunction with variational TST and RRKM calculations. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their singlepoint energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. The calculated potential energy surfaces indicated that energetically the most favorable channel for the HNCC + NO reaction was predicted to be the formation of HNC+CNO (P8) product via the addition reaction of the C atom of HNCC radical and the N atom of NO with the head to head orientation. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for a possible explanation. In addition, the reaction rate constants were calculated using VariFlex code, and the results show that the total rate coefficient, ktotal, at Ar pressure 760 Torr can be represented with an equation: ktotal = 6.433 × 10-11 T0.100 exp(0.275 kcal mol-1/RT) at T = 298-3000 K, in units of cm3 molecule-1 s-1. [ABSTRACT FROM AUTHOR]

Decomposition of energetic material 3,4-dinitropyrazole (DNP) and two model molecules 4-nitropyrazole and 1-nitropyrazole is investigated both theoretically and experimentally. The initial decomposition mechanisms for these three nitropyrazoles are explored with complete active space self-consistent field (CASSCF) level. The NO molecule is observed as an initial decomposition product from all three materials subsequent to UV excitation. Observed NO products are rotationally cold (<50 K) for all three systems. The vibrational temperature of the NO product from DNP is (3850± 50) K, 1350 K hotter than that of the two model species. Potential energy surface calculations at the CASSCF(12,8)/6-31+G(d) level illustrate that conical intersections plays an essential role in the decomposition mechanism. Electronically excited S2 nitropyraozles can nonradiatively relax to lower electronic states through (S2/S1)CI and (S2/S0)CI conical intersection and undergo a nitro-nitrite isomerization to generate NO product either in the S1 state or S0 state. In model systems, NO is generated in the S1 state, while in the energetic material DNP, NO is produced on the ground state surface, as the S1 decomposition pathway is energetically unavailable. The theoretically predicted mechanism is consistent with the experimental results, as DNP decomposes in a lower electronic state than do the model systems and thus the vibrational energy in the NO product from DNP should be hotter than from the model systems. The observed rotational energy distributions for NO are consistent with final structures of the respective transition states for each molecule. [ABSTRACT FROM AUTHOR]

In this work, we present a global potential energy surface for the ground electronic state of the H4+ based on ab initio calculations. The final fit is based on triatomics-in-molecules (TRIM) approximation and it includes extra four-body terms for the better description of some discrepancies found on the TRIM model. The TRIM method itself allows a very accurate description of the asymptotic regions. The global fit uses more than 19 000 multireference configuration interaction ab initio points. The global potential energy surface has an overall root mean square error of 0.013 eV for energies up to 2 eV above the global minimum. This work presents an analysis of the stationary points, reactant and product channels, and crossing between the two lowest TRIM adiabatic states. It is as well included a brief description of the two first excited states of the TRIM matrix, concluding that TRIM method is a very good approximation not only for the ground state but also for at least two of the excited states of H4+ system. [ABSTRACT FROM AUTHOR]

A theoretical investigation of the electronic structure of the NaK molecule including spin-orbit effects has been performed for the 34 Ω(+/-) states dissociating adiabatically into the limits up to Na(3s2S1/2) + K(3d2D3/2) from both an ab initio approach and a long-range model. Equilibrium distances, transition energies, harmonic frequencies as well as depths of wells and heights of humps are reported for all the states. Formulas for calculating the long-range energies for all the 0+/-, 1, 2, and 3 states under investigation are also displayed. They are expressed in terms of the Cn (n = 6,8, ...) long-range coefficients and exchange integrals for the 2S+1Λ(+) parent states, available from literature. As present data could help experimentalists we make available extensive tables of energy values versus internuclear distances in our database at the web address: http://www-lasim.univ-lyon1.fr/spip.php?rubrique99. [ABSTRACT FROM AUTHOR]

The electronic structure of energetically low-lying excited singlet states of fluorobenzene molecules is investigated here. Increasing fluorine substitution alters the nature of the excited electronic states and the so-called perfluoro effect is observed for penta- and hexafluorobenzene. Detailed quantum chemistry calculations are carried out at the equation-of-motion coupled-cluster singles and doubles level of theory to establish the potential energy surfaces of the low-lying electronic states of mono-, di- (ortho- and meta-), and pentafluorobenzene molecules. A sequence of low-energy conical intersections among the electronic potential energy surfaces is established. It is found that increasing fluorine substitution lowers the energy of the πσ* electronic state and leads to conical intersections between the S1 and S2 electronic states of pentafluorobenzene. Existence of numerous conical intersections among the excited electronic states of these molecules forms the mechanistic details underlying their nonradiative internal conversion. In particular, the slow and biexponential fluorescence emission in pentafluorobenzene is attributed to the existence of low-lying S1-S2 conical intersections. The electronic structure data are analyzed in detail and the coupling mechanism among various electronic excited states of mono-, di-, and pentafluorobenzene molecules is established. [ABSTRACT FROM AUTHOR]

Potential energy surface for the lowest quartet state of the rubidium trimer is constructed, making use of the many-body decomposition. Interaction energies are calculated using the coupled-clusters method and interpolated using the reciprocal-power reproducing kernel Hilbert space interpolation method. Both the two-body and three-body nonadditive parts are extrapolated to exhibit the correct long-range behavior. Consequences for the low-energy scattering are briefly discussed. [ABSTRACT FROM AUTHOR]

A new potential energy surface for the electronic ground state of the simplest triatomic anion H3- is determined for a large number of geometries. Its accuracy is improved at short and large distances compared to previous studies. The permanent dipole moment surface of the state is also computed for the first time. Nine vibrational levels of H3- and 14 levels of D3- are obtained, bound by at most ∼70 and ∼126 cm-1, respectively. These results should guide the spectroscopic search of the H3- ion in cold gases (below 100K) of molecular hydrogen in the presence of H- ions. [ABSTRACT FROM AUTHOR]

We report the first measurement of the spectrum of the NO–N2 complex in the region of the first vibrational NO overtone transition. The origin band of the complex is blueshifted by 0.30 cm-1 from the corresponding NO monomer frequency. The observed spectrum consists of three bands assigned to the origin band, the excitation of one quantum of z-axis rotation and one associated hot band. The spacing of the bands and the rotational structure indicate a T-shaped vibrationally averaged structure with the NO molecule forming the top of the T. These findings are confirmed by high level ab initio calculations of the potential energy surfaces in planar symmetry. The deepest minimum is found for a T-shaped geometry on the A″-surface. As a result the sum potential also has the global minimum for this structure. The different potential surfaces show several additional local minima at slightly higher energies indicating that the complex most likely will perform large amplitude motion even in its ground vibrational state. Nevertheless, as suggested by the measured spectra, the complex must, on average, spend a substantial amount of time near the T-shaped configuration. [ABSTRACT FROM AUTHOR]

An approximate coupled-cluster (CC) method—CC singles, doubles, triples, and quadruples involving up to five-pair indices [CCSDTQ(5P)] based on the unrestricted Hartree–Fock (UHF) reference is presented. The key concept is to transform canonical molecular orbitals into corresponding orbitals so that all spin orbitals are grouped into pairs. An approximation to CCSDTQ(5P) is CCSDT(5P), in which none of quadruples are included. These two methods, CCSDT(5P) and CCSDTQ(5P), are approximations to the full CCSDT and CCSDTQ methods, respectively. Both methods computationally scale as the seventh power of the system size. They have been applied to study the bond breaking potential energy surfaces in several closed-shell molecules (HF, F2, CH4, H2O, and N2) and two open-shell molecules (OH and CH3). In comparison with full configuration interaction results, both methods are demonstrated to provide accurate descriptions for single-bond breaking processes, whose performance is significantly better than that of the UHF-based CCSD(T) method. For multiple bond breaking processes in H2O and N2, CCSDTQ(5P) or CCSDT(5P) also provides slightly better results than CCSD(T). [ABSTRACT FROM AUTHOR]

A state-specific vibrational multiconfiguration self-consistent field (VMCSCF) approach based on a multimode expansion of the potential energy surface is presented for the accurate calculation of anharmonic vibrational spectra. As a special case of this general approach vibrational complete active space self-consistent field calculations will be discussed. The latter method shows better convergence than the general VMCSCF approach and must be considered the preferred choice within the multiconfigurational framework. Benchmark calculations are provided for a small set of test molecules. [ABSTRACT FROM AUTHOR]

An approximate coupled cluster singles, doubles, and triples (CCSDT) method based on the unrestricted Hartree–Fock (UHF) reference, in which the contribution of triple excitations is approximately treated in a hybrid manner [denoted as CCSD(T)-h], is presented. In this approach, canonical UHF molecular orbitals are first transformed into corresponding orbitals so that each α-spin orbital is paired with only one β-spin orbital. Then, active orbitals (occupied or virtual) are automatically selected by setting a threshold for the overlap integrals of corresponding orbitals. With the concept of active orbitals, triple excitations can be divided into two subsets: (1) “active” triples involving at least one occupied active orbital and one virtual active orbital and (2) the remaining triples. The amplitudes of these two classes of triple excitations are obtained via two different approaches. When the present method is employed to study bond-breaking processes, it computationally scales as the seventh power of the system size, because the number of active orbitals involved in such processes is relatively small compared to the total number of the orbitals, and is usually independent on the system size. It has been applied to study the bond-breaking potential energy surfaces in the H8 model and five small molecules (HF, F2, CH4, H2O, and N2). For all systems under study, the overall performance of CCSD(T)-h is very competitive with that of CCSDT, and much better than that of the UHF-based CCSD(T). [ABSTRACT FROM AUTHOR]

Transition probabilities on the interaction of the ground and the lowest excited states of gold Au (2S:5d106s1, 2D:5d96s2, and 2P:5d106p1) with silane (SiH4) are studied through ab initio Hartree–Fock self-consistent field calculations, where the atom’s core is represented by relativistic effective core potentials. These calculations are followed by a multiconfigurational self-consistent field study. The correlation energy is accounted for through extensive variational and perturbative second order multireference Moller–Plesset configuration interaction analysis of selected perturbations obtained by iterative process calculations using the CIPSI program package. It is found that the Au atom in the (2P:5d106p1) state inserts in the Si–H bond. In this interaction its corresponding D 2A′ potential energy surface is initially attractive and only becomes repulsive after encountering an avoided crossing with the initially repulsive C 2A′ surface linked to the Au(2D:5d96s2)-SiH4 fragments. The A, B, and C 2A′ curves derived from the Au(2D:5d96s2) atom interaction with silane are initially repulsive, each one of them showing two avoided crossings, while the A 2A′ curve goes sharply downwards until it meets the X 2A′ curve interacting adiabatically, which is linked with the Au(2S:5d106s1)-SiH4 moieties. The A 2A′ curve becomes repulsive after the avoided crossing with the X 2A′, curve. The lowest-lying X 2A′ potential leads to the HAuSiH3 X 2A′ intermediate molecule. This intermediate molecule, diabatically correlated with the Au(2P:5d106p1)+SiH4 system which lies 3.34 kcal/mol above the ground state reactants, has been carefully characterized as have the dissociation channels leading to the AuH+SiH3 and H+AuSiH3 products. These products are reached from the HAuSiH3 intermediate without any activation barrier. The Au–SiH4 calculation results are successfully compared to experiment. Landau–Zener theory of avoided crossings is applied to these interactions considering the angle θ instead of the distance r as the reaction coordinate. [ABSTRACT FROM AUTHOR]

We present a study of the binding energy (BE) curves of rare gas and alkaline-earth dimers using an energy functional that includes exact exchange (EXX) and correlation energies within the random phase approximation (RPA). Our results for the equilibrium positions and long range behavior of the potential energy curves show great improvements over those obtained at the density functional theory level, within local and semilocal approximations. BEs are improved as well in the case of rare gas dimers. For Ar and Kr, the accuracy of our results is comparable to that of so-called van der Waals density functionals, although EXX/RPA yields BE curves that agree better with experiment for large separation distances, as expected. We also discuss shortcomings of the EXX/RPA perturbative approach and analyze possible sources of error in the description of the potential energy curve of alkaline-earth dimers, in particular, Be2, exhibiting an unphysical maximum at large separations. We suggest that the lack of self-consistency in current EXX/RPA approaches might be largely responsible for most of the observed shortcomings. Finally, we present a tight-binding approach to obtain the eigenvalues of the dielectric matrix entering the calculation of the RPA correlation energy that greatly improves the efficiency of EXX/RPA calculations. [ABSTRACT FROM AUTHOR]

Rotational excitation of the O2(X3Σg-) with He is investigated. The calculations are based on the potential energy surface of Groenenboom and Struniewicz [J. Chem. Phys. 113, 9562 (2000)]. Close coupling calculations of the collisional excitation cross sections of the fine-structure levels of O2 by He are calculated for energies up to 2500 cm-1 which yield, after thermal average, rate coefficients up to 350 K. The exact level splitting is taken into account. The propensity rules between fine-structure levels are studied and it is shown that F-conserving cross sections are much larger, especially for high-N rotational levels, than F-changing cross sections, as expected from theoretical considerations. The rate coefficients are almost independent of the rotational quantum number of the O2 molecule for F-conserving transitions. The new rate coefficients can induce important consequences on astrophysical modeling. [ABSTRACT FROM AUTHOR]

Periodic density functional theory calculations are used to systematically investigate, for the first time, the mechanisms for chemical transformations of (R,R)-tartaric acid on a model Cu(110) surface. The overall potential energy surface for the chemical transformations is revealed. The calculations show that the adsorption of the intact biacid molecules of (R,R)-tartaric acid on Cu(110) surface is not strong, but upon adsorption on Cu(110), the biacid molecules will chemically transform immediately, rather than desorb from the surface. It is found that the chemical transformations of (R,R)-tartaric acid on Cu(110) is a thermodynamically favorable process, to produce the monotartrate species, bitartrate species, and H atoms. Kinetically, the initial reaction step is only one O–H bond scission in either one of the COOH group of a biacid molecule of (R,R)-tartaric acid leading to the formation of a monotartrate species and a H atom, which is an almost spontaneous process. The rate-controlling step is the O–H bond scission in the COOH group of a monotartrate species producing a bitartrate species and a H atom. The concerted reaction for simultaneously breaking the two O–H bonds in both COOH groups of a biacid molecule cannot proceed. [ABSTRACT FROM AUTHOR]

High-order many-mode terms in the expansion of multidimensional potential energy surfaces were modeled by a fully automated molecule-specific fitting of parameters within semiempirical molecular orbital theory to low-order grid representations of high-level ab initio potentials. Analytical derivatives of the total energy with respect to the fitting parameters in combination with global and local optimization procedures allow for an efficient and accurate estimation of such terms. The accuracy of this approach was tested on the basis of vibrational SCF and configuration interaction calculations for the fundamental modes of a set of test molecules. Deviations with respect to reference calculations were found to be very small. Speedups in computation time by about three orders of magnitude relative to conventional calculations were achieved. [ABSTRACT FROM AUTHOR]

We calculate the collision-induced, roto-translational, polarized, and depolarized Raman spectra of pairs of H2 molecules. The Schrödinger equation of H2–H2 scattering in the presence of a weak radiation field is integrated in the close-coupled scheme. This permits the accounting for the anisotropy of the intermolecular potential energy surface and thereby it includes mixing of polarizability components. The static polarizability invariants, trace and anisotropy, of two interacting H2 molecules were obtained elsewhere [Li et al., J. Chem. Phys. 126, 214302 (2007)] from first principles. Here we report the associated spherical tensor components which, along with the potential surface, are input in the calculation of the supramolecular Raman spectra. Special attention is paid to the interferences in the wings of the rotational S0(0) and S0(1) lines of the H2 molecule. The calculated Raman pair spectra show reasonable consistency with existing measurements of the polarized and depolarized Raman spectra of pairs of H2 molecules. [ABSTRACT FROM AUTHOR]

Established multidimensional discrete variable representations (DVRs) are derived from a direct product basis. They are commonly used to compute vibrational spectra and have also been employed to determine rovibrational spectra of triatomic molecules. We show that for J>0 calculations the DVR is also advantageous for molecules with more than three atoms. We use a basis of products of Wigner functions (for rotation) and DVR functions (for vibration). A key advantage of the DVR is the fact that one can prune the basis: many DVR functions can be discarded from the original direct product basis. This significantly reduces the cost of the calculation. We have implemented a mapping procedure to exploit this prune-ability. We explain how to treat Coriolis terms in a parity-adapted basis. The method is tested by computing rovibrational levels of HFCO. [ABSTRACT FROM AUTHOR]

By means of a kinetic analysis, we show that the overall rate constant for the collisional loss of orientation or alignment of a rotational level is the sum of the rate constant for elastic depolarization and the sum of the rate constants for all rotationally inelastic transitions out of the level under consideration. An expression for the depolarization cross section is derived in terms of tensor cross sections, and the relationship of depolarization to m-resolved transitions is discussed. We use this formalism in simulations, based on high-quality ab initio potential energy surfaces, of the depolarization of the open-shell molecule OH(X 2Π) through collisions with Ar. Good agreement is seen with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. In addition, we show that the major contribution to elastic collisional depolarization occurs not from weak, glancing collisions but from encounters which probe the inner wall of the potential energy surface. [ABSTRACT FROM AUTHOR]

An accurate full-dimensional global potential energy surface (PES) for the F+CH4→HF+CH3 reaction has been developed based on 19 384 UCCSD(T)/aug-cc-pVTZ quality ab initio energy points obtained by an efficient composite method employing explicit UCCSD(T)/aug-cc-pVDZ and UMP2/aug-cc-pVXZ [X=D,T] computations. The PES contains a first-order saddle point, (CH4- -F)SP, separating reactants from products, and also minima describing the van der Waals complexes, (CH4- - -F)vdW and (CH3- - -HF)vdW, in the entrance and exit channels, respectively. The structures of these stationary points, as well as those of the reactants and products have been computed and the corresponding energies have been determined using basis set extrapolation techniques considering (a) electron correlation beyond the CCSD(T) level, (b) effects of the scalar relativity and the spin-orbit couplings, (c) diagonal Born–Oppenheimer corrections (DBOC), and (d) zero-point vibrational energies and thermal correction to the enthalpy at 298 K. The resulting saddle point barrier and ground state vibrationally adiabatic barrier heights (VSP and VVAGS), dissociation energy of (CH3- - -HF)vdW (De and D0), and the reaction enthalpy (ΔHe°, ΔH0°, and ΔH298°) are (240±40 and 245±200 cm-1), (1070±10 and 460±50 cm-1), and (-10000±50, -11200±80, and -11000±80 cm-1), respectively. Variational vibrational calculations have been carried out for (CH3- - -HF)vdW in full (12) dimensions. Quasiclassical trajectory calculations of the reaction using the new PES are reported. The computed HF vibrational and rotational distributions are in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

We introduce a new reduced-coupling technique to accelerate direct calculations of a selected number of vibrational frequencies in large molecular systems. Our method combines the advantages of the single-to-all correlation-corrected vibrational self-consistent field (STA-CC-VSCF) approach [D. M. Benoit, J. Chem. Phys. 125, 244110 (2006)] with those of the fast-CC-VSCF technique [D. M. Benoit, J. Chem. Phys. 120, 562 (2004)] and allows the ab initio calculation of only the relevant parts of the required potential energy surface (PES). We demonstrate, using a set of five aliphatic alcohol molecules, that the new fast-STA-CC-VSCF method is accurate and leads to very substantial time gains for the computations of the PES. We then use the fast-STA-CC-VSCF method to accelerate the computation of the OH-stretch and NH-stretch frequencies of the two lowest-energy conformers of noradrenaline, namely, AG1a and GG1a. Our new approach enables us to run the calculation 89 times faster than the standard CC-VSCF technique and makes it possible to use a high-level MP2/TZP description of the PES. We demonstrate that the influence of the strong mode-mode couplings is crucial for a realistic description of the particular OH-stretch vibrational signature of each conformer. Finally, of the two possible low-energy conformers, we identify AG1a as the one most likely to have been observed in the experiments of Snoek et al. [Mol. Phys. 101, 1239 (2003)]. [ABSTRACT FROM AUTHOR]

We study the dissociative dynamics of O2 molecules on the Ag(100) surface. Initially, the impinging molecules are either in the spin-triplet ground state or in the spin-singlet excited state. The molecule-surface interaction is obtained in each case by constructing the six-dimensional potential energy surface (PES) from the interpolation of the energies calculated with spin-polarized and non-spin-polarized density functional theories, respectively. Classical trajectory calculations performed in both PESs show that O2 molecules initially in the spin-triplet ground state only dissociate for incidence energies above 1.05 eV. This result is consistent with molecular beam experiments performed in this system. Interestingly, our results also suggest that for the spin-singlet O2 dissociation occurs even for incidence energies as low as 50 meV. We propose the use of spin-singlet excited O2 molecules to improve the otherwise low dissociative reactivity of O2 at clean Ag(100). [ABSTRACT FROM AUTHOR]

An accurate description of electron correlation is essential for the calculation of interaction energies in cases where dispersion energy is a major component, for example, for the rare gas atoms, physisorption on graphite, and graphene-graphene interactions. Such calculations are computationally demanding using supermolecule methods and the energies calculated lack a simple, physical interpretation. Alternatively density functional theories (DFTs) may be used to give an approximate estimate of the correlation energy. However, the physical nature of this DFT estimate of electron correlation energy is not well understood and, in fact, most current DFT methods do not describe dispersion energy at all. Hence, an analysis of the correlation energy contribution to interaction energies where dispersion energy is important is needed. In order to do this we provide an analysis of the correlation energy contribution to the potential energy curves of He2, Ne2, and Ar2 in terms of the Hartree–Fock (HF) interaction term ΔEintHF, a dispersion energy term Edisp and an electron correlation term ΔEintC. ΔEintC includes all other correlation energy effects besides Edisp and is shown to be repulsive, of a similar short range character to, but of smaller magnitude than ΔEintHF. This analysis was used to develop a theoretical model which gives a very good estimate of the potential energy wells for He2, Ne2, Ar2, HeNe, HeAr, and NeAr. [ABSTRACT FROM AUTHOR]

The ground and low-lying states of ozone (O3) have been studied by multireference variational methods and large basis sets. We have constructed potential energy curves along the bending coordinate for (1,2) 1A′, (1,2) 1A″, (1,2) 3A′, and (1,2) 3A″ symmetries, optimizing at the same time the symmetric stretching coordinate. Thirteen minima have been located whose geometrical and energetic characteristics are in very good agreement with existing experimental data. Special emphasis has been given to the interpretation of the chemical bond through valence-bond-Lewis diagrams; their appropriate use captures admirably the bonding nature of the O3 molecule. The biradical character of its ground state, adopted long ago by the scientific community, does not follow from a careful analysis of its wave function. [ABSTRACT FROM AUTHOR]