Search

The potential energy surface describing the interaction of the SH+ ion in its ground X3Σ− electronic state with molecular hydrogen has been computed through restricted coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)], with the assumption of fixed bond lengths. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Representative sets of energy dependent state-to-state rotationally inelastic cross sections and rate coefficients are presented and discussed. A propensity to conserve the fine-structure label is observed, as expected from a formal analysis of the scattering equations. [ABSTRACT FROM AUTHOR]

An effective six-dimensional ab initio potential energy surface (PES) for H2-OCS which explicitly includes the intramolecular stretch normal modes of carbonyl sulfide (OCS) is presented. The electronic structure computations are carried out using the explicitly correlated coupled cluster [CCSD(T)-F12] method with the augmented correlation-consistent aug-cc-pVTZ basis set, and the accuracy is critically tested by performing a series of benchmark calculations. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies to the Morse/long-range potential model. These fits to 13 485 points have a root-mean-square deviation (RMSD) of 0.16 cm-1. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to evaluate the rovibrational energy levels for five isotopic species of the OCS-hydrogen complexes. The predicted transition frequencies and intensities based on the resulting vibrationally averaged PESs are in good agreement with the available experimental values, whose RMSDs are smaller than 0.004 cm-1 for five different species of OCS-hydrogen complexes. The calculated infrared band origin shifts for all five species of OCS-hydrogen complexes are only 0.03 cm-1 smaller than the corresponding experimental values. These validate the high quality of our PESs which can be used for modeling OCS doped in hydrogen clusters to further study quantum solution and microscopic superfluidity. In addition, the analytic coordinate transformation functions between isotopologues are also derived due to the center of mass shifting of different isotope substitutes. [ABSTRACT FROM AUTHOR]

The ring polymer molecular dynamics (RPMD) calculations are performed to calculate rate constants for the title reaction on the recently constructed potential energy surface based on permutation invariant polynomial (PIP) neural-network (NN) fitting [J. Li et al., J. Chem. Phys. 142, 204302 (2015)]. By inspecting convergence, 16 beads are used in computing free-energy barriers at 300 K ≤ T ≤ 1000 K, while different numbers of beads are used for transmission coefficients. The present RPMD rates are in excellent agreement with quantum rates computed on the same potential energy surface, as well as with the experimental measurements, demonstrating further that the RPMDis capable of producing accurate rates for polyatomic chemical reactions even at rather low temperatures. [ABSTRACT FROM AUTHOR]

We have constructed a potential energy surface (PES) for H-atoms interacting with fcc Au(111) based on fitting the analytic form of the energy from Effective Medium Theory (EMT) to ab initio energy values calculated with density functional theory. The fit used input from configurations of the H–Au system with Au atoms at their lattice positions as well as configurations with the Au atoms displaced from their lattice positions. It reproduces the energy, in full dimension, not only for the configurations used as input but also for a large number of additional configurations derived from ab initio molecular dynamics (AIMD) trajectories at finite temperature. Adiabatic molecular dynamics simulations on this PES reproduce the energy loss behavior of AIMD. EMT also provides expressions for the embedding electron density, which enabled us to develop a self-consistent approach to simulate nonadiabatic electron-hole pair excitation and their effect on the motion of the incident H-atoms. For H atoms with an energy of 2.7 eV colliding with Au, electron-hole pair excitation is by far the most important energy loss pathway, giving an average energy loss ≈3 times that of the adiabatic case. This increased energy loss enhances the probability of the H-atom remaining on or in the Au slab by a factor of 2. The most likely outcome for H-atoms that are not scattered also depends prodigiously on the energy transfer mechanism; for the nonadiabatic case, more than 50% of the H-atoms which do not scatter are adsorbed on the surface, while for the adiabatic case more than 50% pass entirely through the 4 layer simulation slab. [ABSTRACT FROM AUTHOR]

We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H+ and H2 molecules. A preliminary study of the reactivity of N2H+ with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H+-H2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of 2530 cm-1. Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H+ and H2 should be very difficult to carry out. To overcome this difficulty, the "adiabatic-hindered-rotor" treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculations. [ABSTRACT FROM AUTHOR]

We present a new four-dimensional potential energy surface for the collisional excitation of HCN by H2. Ab initio calculations of the HCN-H2 van der Waals complex, considering both molecules as rigid rotors, were carried out at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12a] level of theory using an augmented correlation-consistent triple zeta (aVTZ) basis set. The equilibrium structure is linear HCN-H2 with the nitrogen pointing towards H2 at an intermolecular separation of 7.20 a0. The corresponding well depth is -195.20 cm-1. A secondary minimum of -183.59 cm-1 was found for a T-shape configuration with the H of HCN pointing to the center of mass of H2. We also determine the rovibrational energy levels of the HCN-para-H2 and HCN-ortho-H2 complexes. The calculated dissociation energies for the para and ortho complexes are 37.79 cm-1 and 60.26 cm-1, respectively. The calculated ro-vibrational transitions in the HCN-H2 complex are found to agree by more than 0.5% with the available experimental data, confirming the accuracy of the potential energy surface. [ABSTRACT FROM AUTHOR]

We mapped the global three-dimensional potential energy surface (3D-PES) of the water cation at the MRCI/aug-cc-pV5Z including the basis set superposition (BSSE) correction. This PES covers the molecular region and the long ranges close to the H + OH+(X3Σ−), the O + H2+(X2Σg+), and the hydrogen exchange channels. The quality of the PES is checked after comparison to previous experimental and theoretical results of the spectroscopic constants of H2O+(˜X2B1) and of the diatomic fragments, the vibronic spectrum, the dissociation energy, and the barrier to linearity for H2O+(˜X2B1). Our data nicely approach those measured and computed previously. The long range parts reproduce quite well the diatomic potentials. In whole, a good agreement is found, which validates our 3D-PES. [ABSTRACT FROM AUTHOR]

We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ∼35 000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data. [ABSTRACT FROM AUTHOR]

A new analytical potential energy surface is presented for the reaction of hydrogen abstraction from methane by a hydrogen atom. It is based on an analytical expression proposed by Jordan and Gilbert [J. Chem. Phys. 102, 5669 (1995)], and its fittable parameters were obtained by a multibeginning optimization procedure to reproduce high-level ab initio electronic structure calculations obtained at the CCSD(T)/cc-pVTZ level. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath. No experimental information is used. By comparison with the reference results we show that the resulting surface reproduces well not only the ab initio data used in the fitting but also other thermochemical and kinetic results computed at the same ab initio level, such as equilibrium constants, rate constants, and kinetic isotope effects, which were not used in the fit. In this way we show that the new potential energy surface is correctly fitted and almost as accurate as the CCSD(T)/cc-pVTZ method in describing the kinetics of the reaction. We analyze the limitations of the functional form and the fitting method employed, and suggest some solutions to their drawbacks. In a forthcoming communication, we test the quality of the new surface by comparing its results with experimental values. [ABSTRACT FROM AUTHOR]

The infrared spectrum of the Al+–H2 complex is recorded in the H–H stretch region (4075–4110 cm-1) by monitoring Al+ photofragments. The H–H stretch band is centered at 4095.2 cm-1, a shift of -66.0 cm-1 from the Q1(0) transition of the free H2 molecule. Altogether, 47 rovibrational transitions belonging to the parallel Ka=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al+–H2 has a T-shaped equilibrium configuration with the Al+ ion attached to a slightly perturbed H2 molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 Å, decreasing by 0.03 Å when the H2 subunit is vibrationally excited. A three-dimensional potential energy surface for Al+–H2 is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al+–H2(para) and Al+–H2(ortho) are predicted, respectively, to be 469.4 and 506.4 cm-1, in good agreement with previous measurements. The calculations reproduce the experimental H–H stretch frequency to within 3.75 cm-1, and the calculated B and C rotational constants to within ∼2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum. [ABSTRACT FROM AUTHOR]

A global analytical potential energy surface for the ground state of H3- has been constructed by fitting an analytic function to the ab initio potential energy values computed using coupled cluster singles and doubles with perturbative triples [CCSD(T)] method and Dunning’s augmented correlation consistent polarized valence triple zeta basis set. Using this potential energy surface, time-dependent quantum mechanical wave packet calculations were carried out to calculate the reaction probabilities (PR) for the exchange reaction H-+H2(v,j)→H2+H-, for different initial vibrational (v) and rotational (j) states of H2, for total angular momentum equal to zero. With increase in v, the number of oscillations in the PR(E) plot increases and the oscillations become more pronounced. While PR increases with increase in rotational excitation from j=0 to 1, it decreases with further increase in j to 2 over a wide range of energies. In addition, rotational excitation quenches the oscillations in PR(E) plots. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a full dimensional, ab initio based potential energy surface for CH5+. The ab initio electronic energies and gradients are obtained in direct-dynamics calculations using second-order Møller-Plesset perturbation theory with the correlation consistent polarized valence triple zeta basis. The potential energy and the dipole moment surfaces are fit using novel procedures that ensure the full permutational symmetry of the system. The fitted potential energy surface is tested by comparing it against additional electronic energy calculations and by comparing normal mode frequencies at the three lowest-lying stationary points obtained from the fit against ab initio ones. Well-converged diffusion Monte Carlo zero-point energies, rotational constants, and projections along the CH and HH bond lengths and the tunneling coordinates are presented and compared with the corresponding harmonic oscillator and standard classical molecular dynamics ones. The delocalization of the wave function is analyzed through comparison of the CH5+ distributions with those obtained when all of the hydrogen atoms are replaced by 2H and 3H. The classical dipole correlation function is examined as a function of the total energy. This provides a further probe of the delocalization of CH5+. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new highly accurate interaction potential is constructed for the He-H2 van der Waals complex. This potential is fitted to 1900 ab initio energies computed at the very large-basis coupled-cluster level and augmented by corrections for higher-order excitations (up to full configuration interaction level) and the diagonal Born-Oppenheimer correction. At the vibrationally averaged H-H bond length of 1.448736 bohrs, the well depth of our potential, 15.870 ± 0.065 K, is nearly 1 K larger than the most accurate previous studies have indicated. In addition to constructing our own three-dimensional potential in the van der Waals region, we present a reparameterization of the Boothroyd-Martin-Peterson potential surface [A. I. Boothroyd, P. G. Martin, and M. R. Peterson, J. Chem. Phys. 119, 3187 (2003)] that is suitable for all configurations of the triatomic system. Finally, we use the newly developed potentials to compute the properties of the lone bound states of 4He-H2 and 3He-H2 and the interaction second virial coefficient of the hydrogen-helium mixture. [ABSTRACT FROM AUTHOR]

We report the properties of two novel transition states of the bimolecular hydrogen exchange reaction in the water dimer, based on an ab initio water dimer potential [A. Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The realism of the two transition states is assessed by comparing structures, energies, and harmonic frequencies obtained from the potential energy surface and new high-level ab initio calculations. The rate constant for the exchange is obtained using conventional transition state theory with a tunneling correction. We employ a one-dimensional approach for the tunneling calculations using a relaxed potential from the full-dimensional potential in the imaginary-frequency normal mode of the saddle point, Qim. The accuracy of this one-dimensional approach has been shown for the ground-state tunneling splittings for H and D-transfer in malonaldehyde and for the D+H2 reaction [Y. Wang and J. M. Bowman, J. Chem. Phys. 129, 121103 (2008)]. This approach is applied to calculate the rate constant for the H2O+H2O exchange and also for H2O+D2O→2HOD. The local zero-point energy is also obtained using diffusion Monte Carlo calculations in the space of real-frequency-saddle-point normal modes, as a function of Qim. [ABSTRACT FROM AUTHOR]

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcal/mol, in excellent agreement with the reported ab initio value. Model one-dimensional and “exact” full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased “fixed-node” diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm-1 in Cartesian coordinates and 22.6 cm-1 in normal coordinates, with an uncertainty of 2–3 cm-1. This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21–22 cm-1. The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2–3 cm-1. These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm-1, and agree well with the experimental values of 21.6 and 2.9 cm-1 for the H and D transfer, respectively. [ABSTRACT FROM AUTHOR]

Detailed quasiclassical trajectory calculations of the reaction H+CH4(ν3=0,1)→CH3+H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [X. Zhang et al., J. Chem. Phys. 124, 021104 (2006)] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C–H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0±1.5 [J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005)]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories. [ABSTRACT FROM AUTHOR]

A modified and recalibrated potential energy surface for the gas-phase Cl+CH4→HCl+CH3 reaction is reported and tested. It is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to updated experimental and theoretical stationary point properties and experimental forward thermal rate constants. From the kinetics point of view, the forward and reverse thermal rate constants and the activation energies were calculated using the variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 150–2500 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of tunneling in this hydrogen abstraction reaction. A dynamics study was also performed on this PES using quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found a noticeable internal energy in the coproduct methyl radical, both in the ground-state [CH4 (v=0)] and vibrationally excited [CH4 (v=1)] reactions. This CH3 internal energy was directly precluded in some experiments or oversimplified in previous theoretical studies using pseudotriatomic models. Second, our QCT calculations give HCl rotational distributions slightly hotter than those in experiment, but correctly describing the experimental trend of decreasing the HCl product rotation excitation in going from HCl (v′=0) to HCl (v′=1) for the CH4 (v=1) reaction. Third, the state specific scattering distributions present a reasonable agreement with experiment, although they tend to make the reaction more forward and backward scattered than found experimentally probably because of the hotter rotational distribution and the deficiencies of the QCT methods. [ABSTRACT FROM AUTHOR]

Classical simulations of the reactions between HCO+/COH+ and hydrogen atoms, as well as their deuterated variants, have been carried out on an ab initio interpolated potential energy surface. The surface is constructed at the quadratic configuration interaction with single and double excitation level of ab initio calculation. At low energies we observe reaction channels associated with the isomerization of the cation, hydrogen/deuterium exchange, and the combination of isomerization with exchange. The HCO+/DCO+ ions only undergo exchange, and deuteration is more facile than the release of deuterium. The COH+/COD+ ions undergo isomerization or isomerization combined with exchange, the latter being the dominant reaction channel. Deuteration is again more facile than the release of deuterium, in combination with isomerization. These results are consistent with experimental measurements and with hypotheses on the deuteration of molecules in the interstellar medium. [ABSTRACT FROM AUTHOR]

A two-dimensional (rigid monomer) intermolecular potential energy surface (PES) of the He–HCl complex has been obtained from ab initio calculations utilizing the symmetry-adapted perturbation theory (SAPT) and an spdfg basis set including midbond functions. The bond length in HCl was chosen to be equal to the expectation value in the ground vibrational state of isolated HCl. The rigid-monomer potential should be a very good approximation to the complete (three-dimensional) potential for H–Cl distances corresponding to the lowest vibrational levels of the monomer since the He–HCl interaction energy was found to be only weakly dependent on the HCl bond length in this region, at least as compared to systems such as Ar–HF. The calculated points were fitted using an analytic function with ab initio computed asymptotic coefficients. As expected, the complex is loosely bound, with the dispersion energy providing the majority of the attraction. Our SAPT PES agrees with the semiempirical PES of Willey et al. [J. Chem. Phys. 96, 898 (1992)], in finding that, atypically for rare gas–hydrogen halide complexes including the lighter halide atoms, the global minimum is on the Cl side (with intermonomer separation 3.35 Å and depth of 32.8 cm-1), rather than on the H side, where there is only a local minimum (3.85 Å, 30.8 cm-1). The ordering of the minima was confirmed by single-point calculations in larger basis sets and complete basis set extrapolations, and also using higher levels of theory. We show that the opposite findings in the recent calculations of Zhang and Shi [J. Mol. Struct: THEOCHEM 589, 89 (2002)] are due to the lack of midbond functions in their basis set. Despite the closeness in depth of the two linear minima, the existence of a relatively high barrier between them invalidates the assumption of isotropy, a feature of some literature potentials. The trends concerning the locations of minima within the family of rare gas–hydrogen halide complexes are rationalized in terms of the physical components of the intermolecular forces and related to monomer properties. The accuracy of the SAPT PES was tested by performing calculations of rovibrational levels. The transition frequencies obtained were found to be in excellent agreement (to within 0.02 cm-1) with the measurements of Lovejoy and Nesbitt [J. Chem. Phys. 93, 5387 (1990)]. The SAPT PES predicts a dissociation energy for the complex of 7.74 cm-1 which is probably more accurate than the experimental value of 10.1±1.2 cm-1. Our analysis of the ground-state rovibrational wave function shows that the He–HCl configuration is favored over the He–ClH configuration despite the ordering of minima. This is due to the greater volume of the well in the former case. We have also determined positions and widths of three low-lying resonance states through scattering calculations. These predictions are expected to be more accurate than values derived from experiment.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new potential energy surface for the O(3P)+H2 system in the lowest 3A″ state is built using ab initio data calculated by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)] and the double many-body expansion formalism. It incorporates a semiempirical model of long-range interactions, which should play an important role at low collision energies. Preliminary quasiclassical trajectory results at 12.6 kcal/mol collision energy, show that the deeper van der Waals region described in this new surface translates into a four times higher cross section than that of Rogers’ 3A″ surface. To confirm this hypothesis, a second surface was calibrated. The two surfaces are fitted with rmsd<0.5 kcal/mol and differ mainly on the depth of the van der Waals region. That difference in the van der Waals region corresponds to a 22% lower cross section of the less deep surface, which is still three times higher than the equivalent results from Rogers’ 3A″ surface. This study reflects the importance of a correct description of van der Waals forces on potential energy surfaces. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density-functional theory calculations based on plane-wave expansion and pseudopotential treatment were carried out for atomic hydrogen on a rigid Cu(100) surface. A global potential energy surface was then obtained by using a three-dimensional spline interpolation. It is found that the minimum of the potential is located at the fourfold hollow site with a diffusion barrier of 88 meV at the bridge site. The vibrational states of atomic hydrogen and deuterium on the Cu(100) surface were calculated on the potential surface. Our calculations show that the vibrational states A10, A11, E1, and B21 of H/Cu(100) exhibit strong localized character and very narrow band widths, whereas other excited vibrational states have considerable delocalized character and broad band widths. The vibrational frequency of 71.2 (51.5) meV for H(D) in the perpendicular direction obtained in this study is in good agreement with the experimentally observed value of 70 (52) meV [Surf. Sci. 248, 35 (1991)]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A potential energy surface for the ground electronic state of the Ar–HI van der Waals complex is calculated at the coupled-cluster with single and double excitations and a noniterative perturbation treatment of triple excitations [CCSD(T)] level of theory. Calculations are performed using for the iodine atom a correlation consistent triple-ζ valence basis set in conjunction with large-core Stuttgart–Dresden–Bonn relativistic pseudopotential, whereas specific augmented correlation consistent basis sets are employed for the H and Ar atoms supplemented with an additional set of bond functions. In agreement with previous studies, the equilibrium structure is found to be linear Ar–I–H, with a well depth of 205.38 cm-1. Another two secondary minima are also predicted at a linear and bent Ar–H–I configurations with well depths of 153.57 and 151.57 cm-1, respectively. The parametrized CCSD(T) potential is used to calculate rovibrational bound states of Ar–HI/Ar–DI complexes, and the vibrationally averaged structures of the different isomers are determined. Spectroscopic constants are also computed from the CCSD(T) surface and their comparison with available experimental data demonstrates the quality of the present surface in the corresponding configuration regions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–C2H2 complex. The interaction energy is found to be dominated by the first-order exchange contribution and the dispersion energy. In both contributions it was necessary to include high-level intramolecular correlation effects. Our potential has a global minimum of εm=-22.292 cm-1 near the linear He–HCCH geometry at Rm=8.20 bohr and [variant_greek_theta]m=14.16°, and a local minimum at a skew geometry (Rm=7.39 bohr, [variant_greek_theta]m=48.82°, and εm=-21.983 cm-1). The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–C2H2 molecule and the near-infrared spectrum, which corresponds to the simultaneous excitation of the vibration and hindered rotation of the C2H2 monomer within the complex. The nature of the bound states and of the spectrum predicted from the ab initio potential are discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An accurate potential energy surface of the He–H2 interaction is calculated with a large basis set at the complete fourth-order Mo\ller–Plesset approximation. The basis set—a combination of a nucleus-centered set 6s4p2d and a bond function set 3s3p2d centered at the midpoint between He and the H2 center of mass—is designed to give the optimal description of both the intra- and intersystem correlation effects. The validity of the basis set is confirmed by extensive preliminary calculations on the linear (orientation angle θ=0°), bent (45°), and T-shaped (90°) structures at a fixed separation (R=6.5a0) with a series of large basis sets containing different polarization functions and/or bond functions. Bond functions are found more effective than polarization functions in recovering the intersystem correlation energy and they are particularly useful in removing the geometric bias of a basis to give an accurate description for the potential anisotropy and the relative energies of different structures. The effect of bond functions is insensitive to the displacement of bond functions and the geometric midpoint of the van der Waals bond is a satisfactory choice for the center of bond functions. The potential energy surface of He–H2 is calculated at 15 values of R from 2.0 to 15.0a0 along each of the three main configurations (θ=0°, 45°, and 90°) with the vibrationally averaged H2 bond length r=1.449a0. Additional calculations are given for r=1.28 and 1.618a0 to show the effect of H2 zero-point vibration. While our potential at the self-consistent field (SCF) level is essentially the same as the previous calculations, our potential at the correlated level is globally deeper in the attractive region and less repulsive in the shorter range. Our calculated well depth (47.19 μhartrees) corresponding to the global minimum at θ=0° and R=6.5a0, is very close to the estimated experimental... [ABSTRACT FROM AUTHOR]

Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H–H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75±3μEh. Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces (25–70 kcal/mol above the H–H2 asymptote) at small interatomic separations; the Boothroyd, Keogh, Martin, and Peterson (BKMP) potential energy surface is found to agree with results of the present calculations to within the expected uncertainty (±1 kcal/mol) of the fit. Multipolar expansions of the computed H–H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a0) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment. [ABSTRACT FROM AUTHOR]

The many-body symmetry adapted perturbation theory has been applied to compute the Ar–H2 potential energy surface. Large basis sets containing spdfgh-symmetry orbitals optimized for intermolecular interactions have been used to achieve converged results. For a broad range of the configuration space the theoretical potential energy surface agrees to almost two significant digits with the empirical potential extracted from scattering and infrared spectroscopy data by Le Roy and Hutson. The minimum of our theoretical potential is εm=-164.7 cal/mol and is reached at the linear geometry for the Ar–H2 distance Rm=6.79 bohr. These values agree very well with corresponding empirical results εm=-161.9 cal/mol and Rm=6.82 bohr. For the first time such a quantitative agreement has been reached between theory and experiment for a van der Waals system that large. Despite such excellent agreement in the overall potential, the exponential and the inverse R components of it agree to only about 20%. [ABSTRACT FROM AUTHOR]

Extensive ab initio calculations have been performed on the reaction N+3(P)+H21(Σ+g)NH+ (X 2Π)+H(2S), using complete-active-space self-consistent-field and multireference contracted configuration interaction methods. The 752 calculated energy points belonging to the 1 3A‘ surface were fitted to an analytical potential energy surface, with an average least-squares deviation of only 0.026 eV (2.5 kJ/mol). A key to this accurate fit of the surface, which does include minima as deep as 6 eV, is the use of R-n polynomials in the asymptotic region, and Rn polynomials in the deep minima regions. The calculated reaction endothermicity is 0.03 eV, after correcting for the differences in zero point vibrational energies. This is in agreement with the essentially thermoneutral reaction enthalpy, obtained in recent experiments. An important feature of the potential energy surface is the anisotropy in the reactant channel, arising from the ion–quadrupole interaction. [ABSTRACT FROM AUTHOR]

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

The strong FDBr interaction region located in the corner of the LEPS model potential energy surface is diagnosed as dynamical white spot. Although energetically accessible, it is dynamically unapproachable by the vast majority of reactive classical trajectories, quantum waves represented by S-matrix propagation, or resonance wave functions. The quantum and classical derivations and explanations yield important consequences of the dynamical white spot: In particular, quantum chemical evaluations of this region of the potential energy surface are rendered unneccessary because its dynamical effects on bi- and unimolecular reactivity are insignificant. [ABSTRACT FROM AUTHOR]

The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star-forming regions. We report here a full nine-dimensional interaction potential for H2O-H2 calibrated using high-accuracy, explicitly correlated wave functions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the υ2 bending mode of H2O obtained from quasiclassical trajectory calculations in the temperature range of 500–4000 K. Our high-temperature (T>=1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough reinterpretation of vibrationally excited water emission spectra from space. [ABSTRACT FROM AUTHOR]

We report on a global potential energy hypersurface for the lowest triplet state of H[sup +, sub 3] based on the double many-body expansion (DMBE) method and ab initio data points of V5Z quality. These data points are represented with a root mean square deviation of only 3.46 cm[sup -1] in the energy region dissociation threshold and with less than 15 cm[sup -1] up to the below the H[sup +, sub 2] (X²Σ[sup +, sub g]) + H(²S) three-particle breakup energy. As expected from a DMBE approach, all dissociation limits are represented accurately and are approached in a physically correct manner. [ABSTRACT FROM AUTHOR]

Describes a first-principles potential energy surface for Eley-Rideal reaction dynamics of hydrogen atoms on Cu(111). Details of the total-energy calculations; Model potential energy surfaces; Results of calculations of the total energy for various configurations of two H atoms on the Cu(111) surface.

Develops a potential energy surface (PES) for the gas-phase germanium-hydrogen compound reaction and its deuterated analog. Inclusion of suitable functional forms to represent the stretching and bending modes; Calculation of thermal rate constants using variational transition state theory with semiclassical transmission coefficients.

The influence of non-adiabatic effects on the ro-vibrational bound states of H+3 has been investigated using geometry-dependent reduced masses and only one single potential energy surface. The used potentials (BO electronic energy, adiabatic corrections and relativistic contributions) are based on explicitly correlated wavefunctions. For the first time, several different fully geometry-dependent reduced mass surfaces in three dimensions have been incorporated for the vibrational and rotational contributions. [ABSTRACT FROM AUTHOR]

The potential energy surface of the HO2+NO reaction has been investigated at second order Moller–Plesset perturbation (MP2) and density functional (DFT) methods with the 6-311++G** basis set and at complete active space [CAS(8,8)] self-consistent field level using the 6-31G** basis set. The reaction is shown to give three different groups of products, viz., HO–NO2, NO2+OH, and HNO+O2. The thermodynamically stable HO–NO2 can be formed from the energized ONOOH adduct by the 1,2 migration of the OH group via a loose transition state (referred to as TS2) with a relatively higher barrier height compared to O–O bond fission. The other exothermic product, NO2+OH, arises from a direct O–O dissociation of ONOOH and is expected to be the most favorable process on account of its low barrier height. HNO+O2 can be formed by two different channels: (i) the direct hydrogen abstraction and/or (ii) the barrierless association of the reactants to form the peroxynitrous acid, ONOOH, which then undergoes 1,3 hydrogen migration, giving rise to the HN(O)OO biradical followed by N–O dissociation. Of the two channels, channel (i) has been found to be dominant. Owing to their higher barrier heights, HNO formation is expected only at high temperatures. NOH+O2 and HONO+O are not expected to compete in the kinetics of the HO2+NO system. The energetic of the key reactions, namely HO2+NO→HO–NO2 and HO2+NO→NO2+OH, has also been obtained at the QCISD/6-311++G(2df,2pd)//MP2/6-311++G** level. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Applies the symmetry-adapted perturbation theory to compute the intermolecular potential energy surface of the argon-carbon-hydrogen complex. Dominance of first-order exchange contribution in the interaction energy; Ab initio potential having four equivalent minima for structures in which the argon atom approaches the face of tetrahedron.

We present a potential energy surface for the CH4+Cl→CH3+Cl reaction, based on the analytical function J1 for the analog CH4+H→CH3+H2 reaction by Joseph et al. To calibrate the new surface we chose the reactant and product experimental properties as reference data. The forward and reverse rate constants were calculated using variational transition state theory with large curvature transmission coefficients over a wide temperature range, 200–1000 K. The variational effects were concluded to be small for this reaction, and good agreement with experimental rate constants was found in both forward and reverse reactions. The kinetic isotope effects (KIEs) at different temperatures for the forward and reverse reactions were also analyzed showing always a ‘‘normal’’ behavior. The factor analysis of the KIEs in the forward reactions indicated high vibrational and tunneling contributions at low temperatures. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In evaluating some low temperature (T<1000 K) thermal rate coefficients for inelastic rotational excitation of H2 by H atoms, Sun and Dalgarno have found a marked sensitivity to the potential energy surface adopted for the calculations. We have investigated the origin of the discrepancies between previous H3 potential energy surfaces and have developed a refined surface which addresses these concerns. New quasiclassical trajectory calculations of cross sections for low energy rotational excitation are reported. The refined surface is based on 8701 ab initio energies, most newly computed for this purpose. It has the same functional form as our earlier (BKMP) surface, but since the fit of the parameters is more fully constrained than for any previous surface it is a more accurate representation. The refined surface matches the ab initio energies with an overall rms error of 0.27 mEh (i.e., 0.17 kcal/mol) and a maximum absolute deviation of 6.2 mEh (for a very compact high energy equilateral triangle conformation). For ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.18 mEh and the maximum absolute deviation is 1.7 mEh. The refined surface is compared critically to four previous surfaces, including the DMBE surface of Varandas et al., in several respects: Legendre expansion coefficients; the interaction region for low energy rotational excitation; near the collinear saddle point; near conical intersections of the ground and first excited state surfaces; the van der Waals well; and compact geometries. We have also compared new first excited state ab initio energies for 1809 conformations with corresponding predictions from the DMBE surface. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A six-dimensional (6D) potential energy surface (PES) describing the molecule–surface interaction in the dissociative chemisorption system H2+Cu(100) is presented. The PES is based on slab calculations performed using the generalized gradient approximation (GGA) of density functional theory (DFT). To allow the use of the PES in dynamics calculations which can test the validity of the DFT/slab approach by comparing with available experiments on dissociative chemisorption, the PES was fit to an analytical form. The fit used describes the orientational dependence of the molecule–surface interaction above the high symmetry sites upto second order in spherical harmonics. The barriers to dissociation calculated for H2 approaching with its molecular axis parallel to the surface are all located in the exit channel. Also, for different impact sites and orientations, the height and the distance to the surface associated with the barrier correlate well with the chemisorption energy of the H-atoms in the sites to which dissociation takes place; the lowest barrier (0.48 eV) is found for dissociation over the bridge site into the hollow sites, the atomic chemisorption energy being highest in the hollow sites. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A two-dimensional (2-D) potential energy surface (PES) has been calculated for H2 interacting with the (100) face of copper. The PES is for H2 approaching with its internuclear axis parallel to the surface and dissociating over a bridge site into neighboring hollow sites. The density functional calculations were performed both within the local density approximation (LDA) and within a generalized gradient approximation (GGA). The LDA surface shows no barrier to chemisorption, but the GGA surface has a barrier of height 0.4 eV. A fit of the GGA surface has been used to calculate reaction probabilities for H2 in its v=0 and v=1 vibrational states, employing a wave packet method. The 2-D wave packet results for the v=0 and v=1 thresholds are consistent with experiment, indicating that the barrier height calculated within the GGA used is accurate. The GGA results for the value of the barrier height are also consistent with the GGA value (0.5 eV) recently obtained for H2+Cu(111) by Hammer et al. [Phys. Rev. Lett. 73, 1400 (1994)], but the GGA value recently computed for H2+Cu(100) (0.9 eV) by White et al. is too high [Phys. Rev. Lett. 73, 1404 (1994)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6-31G(d) level. GAUSSIAN-2 theory was employed to calculate ΔHf,298 for HSO and HOS of -19.9 and -5.5 kJ mol-1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10-16 (T/K)1.746 exp(-1457 K/T) cm3 molecule-1 s-1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low-barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and HSO and HOS+H→H2+3SO are discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) for the adiabatic ground state of the H4 system which fit published ab initio data [Boothroyd, et al. J. Chem. Phys. 95, 4331 (1991)] at a quantitative level has been obtained (root-mean-square error about 2 mhartree or 1–2 kcal/mol), and without any quantity of ad hoc character, preserving the accuracy of the ab initio points. The global fitting procedure used here is an extension of the corresponding procedure for triatomic systems including the functional form previously proposed by the authors. The global H4 PES obtained here is totally symmetric with respect to permutations of the hydrogen atoms and satisfies the criteria needed to be used in scattering calculations. [ABSTRACT FROM AUTHOR]

We report ‘‘exact’’ quantum Monte Carlo calculations of the potential energy surface for the reaction H+H2→H2+H. The method used is free of systematic error. The statistical or sampling error was reduced to ±0.10 kcal/mol for several hundred points distributed across the surface, to ±0.02 kcal/mol for the minimum energy approach of H to H2, to ±0.02 kcal/mol near the saddle point, and to ±0.01 kcal/mol at the saddle point. The upper and lower surfaces in the region of the Jahn–Teller cusp were determined with a statistical error of ±0.2 kcal/mol. [ABSTRACT FROM AUTHOR]

Three different functional forms are fit to a calculated coupled electron pair approach potential energy surface for the reaction Ne+H2+→NeH++H. Minimum energy pathways and stationary points of the various fits are discussed. [ABSTRACT FROM AUTHOR]

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data. [ABSTRACT FROM AUTHOR]

Two findings are reported for the D+H2→DH+H reaction on the basis of the exact quantum mechanical calculation for J=0, where J is total angular momentum. First, with use of the Liu–Siegbahn–Truhlar–Horowitz (LSTH) surface and the Varandas surface, we demonstrate that a rather small difference in potential energy surface (PES) induces a surprisingly large effect on reaction dynamics. Two origins of the discrepancy are pointed out and analyzed: (1) Noncollinear conformation in the reaction zone contributes to the reaction significantly despite the fact that the minimum energy path and the saddle point are located in the collinear configuration. (2) A difference in the distant part of PES also causes a discrepancy in the reaction dynamics indirectly, although this effect is much smaller than (1). Secondly, we investigate the validity of the constant centrifugal potential approximation (CCPA) based on the accurate results for J=0. The use of CCPA to estimate total cross section and rate constant is again proved to have practical utility as in the cases of the sudden and adiabatic approximations. [ABSTRACT FROM AUTHOR]

We report ab initio calculations of the ground state energy for 404 new conformations of H3, supplementing the set of 368 conformations reported previously by others. The entire dataset has been used to constrain an analytical functional form for the potential energy surface, building on that of Truhlar and Horowitz. The new surface extends the Truhlar and Horowitz surface to higher energies and offers some modest improvement at lower energies. In addition, we have eliminated a problem with derivatives of the London equation that was pointed out by Johnson. The new surface matches the 772 ab initio energies with an overall root-mean-square (rms) error of 0.25 mhartree (i.e., 0.16 kcal/mol) and a maximum absolute deviation of 1.93 mhartree (1.21 kcal/mol); for ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.17 mhartree (0.11 kcal/mol) and the maximum absolute deviation is 1.10 mhartree (0.69 kcal/mol). The classical barrier height for H+H2→H2+H is estimated to be 15.20±0.15 mhartree (i.e., 9.54±0.09 kcal/mol). [ABSTRACT FROM AUTHOR]

We investigated the potential energy surface for the H2O···H interaction in the van der Waals well region. Calculations were carried out using the Mo\ller–Plesset second- and fourth-order perturbation theory in a [12s,7p,2d]→(6s,5p,2d) basis set for the O atom, and [6s,2p,1d]→(5s,2p,1d) for the H atoms. Basis set and superposition error effects were analyzed to gauge the reliability of the calculated potential. The potential was investigated in five physically distinct directions. The deepest potential well was found in the H2O molecular plane 3.30–3.45 Å from the H2O center of mass, near the H end of the OH bond. The following parameters are suggested for the spherically averaged potential: well depth 53±6 cm-1; minimum distance from the center of mass 3.25–3.40 Å. [ABSTRACT FROM AUTHOR]

The reactive cross sections and product angular distributions for the F+H2,F+D2 and F+HD reactions have been calculated using the infinite-order-sudden approximation on a modified London–Eyring–Polanyi–Sato potential energy surface which has a nonlinear saddle point. This surface was constructed previously so as to reproduce the experimentally obtained product angular distributions by the quasiclassical trajectory calculations. The calculated branching ratios of different vibrational states of products, HF(v’) and DF(v’) from above three reactions, were all in qualitative agreement with those experimentally obtained; however, the product angular distributions calculated were not better than those calculated by the quasiclassical trajectory method. These results are compared with those calculated on different potential surfaces which predict collinear transition states. [ABSTRACT FROM AUTHOR]

In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state-to-state cross sections in both the dissociated and the scattered flux. The potential energy surface is two-dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule. [ABSTRACT FROM AUTHOR]