Theoretical study of the dynamics of AR collisions with C2H6 and C2F6 at hyperthermal energy

We present a classical-trajectory study of the dynamics of high-energy (5-12 eV) collisions between Ar atoms and the C2H6 and C2F6 molecules. We have constructed the potential-energy surfaces for these systems considering separately the Ar-molecule interactions (intermolecular potential) and the interactions within the molecule (intramolecular potential). The intermolecular surfaces consist of pairwise empirical potentials derived from high-accuracy ab initio calculations. The intramolecular potentials for C2H6 and C2F6 are described using specific-reaction-parameters semiempirical Hamiltonians and are calculated "on the fly", i.e., while the trajectories are evolving. Trajectory analysis shows that C2F6 absorbs more energy than C2H6 and is more susceptible to collision-induced dissociation (CID). C-C bond-breakage processes are more important than C-H or C-F bond breakage at the energies explored in this work. Analysis of the reaction mechanism for CID processes indicates that, although C-C breakage is mostly produced by side-on collisions, head-on collisions are more efficient in producing C-F or C-H dissociation. Our results suggest that high-energy collisions between closed-shell species of the natural low-Earth-orbit environment and spacecraft can contribute to the observed degradation of polymers that coat spacecraft surfaces.     

New exchange-Coulomb N2-Ar potential-energy surface and its comparison with other recent N2-Ar potential-energy surfaces

The reliability of five N2-Ar potential-energy surfaces in representing the N2-Ar interaction has been investigated by comparing their abilities to reproduce a variety of experimental results, including interaction second viral coefficients, bulk transport properties, relaxation phenomena, differential scattering cross sections, and the microwave and infrared spectra of the van der Waals complexes. Four of the surfaces are the result of high-level ab initio quantal calculations; one of them utilized fine tuning by fitting to microwave data. To date, these four potential-energy surfaces have only been tested against experimental microwave data. The fifth potential-energy surface, based upon the exchange-Coulomb potential-energy model for the interaction of closed-shell species, is developed herein: it is a combination of a damped dispersion energy series and ab initio calculations of the Heitler-London interaction energy, and has adjustable parameters determined by requiring essentially simultaneous agreement with selected quality interaction second viral coefficient and microwave data. Comparisons are also made with the predictions of three other very good literature potential-energy surfaces, including the precursor of the new exchange-Coulomb potential-energy surface developed here. Based upon an analysis of a large body of information, the new exchange-Coulomb and microwave-tuned ab initio potential-energy surfaces provide the best representations of the N2-Ar interaction; nevertheless, the other potential-energy surfaces examined still have considerable merit with respect to the prediction of specific properties of the N2-Ar van der Waals complex. Of the two recommended surfaces, the new exchange-Coulomb surface is preferred on balance due to its superior predictions of the effective cross sections related to various relaxation phenomena, and to its reliable, and relatively simple, representation of the long-range part of the potential-energy surface. Moreover, the flexibility still inherent in the exchange-Coulomb potential form can be further exploited, if required, in future studies of the N2-Ar interaction.     

Potential-energy surface for the electronic ground state of NH3 up to 20,000 cm-1 above equilibrium

Ab initio coupled cluster calculations with single and double substitutions and a perturbative treatment of connected triple excitations [CCSD(T)] with the augmented correlation-consistent polarized valence triple-zeta aug-cc-pVTZ basis at 51 816 geometries provide a six-dimensional potential-energy surface for the electronic ground state of NH3. At 3814 selected geometries, CBS+ energies are obtained by extrapolating the CCSD(T) results for the aug-cc-pVXZ(X=T,Q,5) basis sets to the complete basis set (CBS) limit and adding corrections for core-valence correlation and relativistic effects. CBS** ab initio energies are generated at 51,816 geometries by an empirical extrapolation of the CCSD(T)/aug-cc-pVTZ results to the CBS+ limit. They cover the energy region up to 20,000 cm-1 above equilibrium. Parametrized analytical functions are fitted through the ab initio points. For these analytical surfaces, vibrational term values and transition moments are calculated by means of a variational program employing a kinetic-energy operator expressed in the Eckart-Sayvetz frame. Comparisons against experiment are used to assess the quality of the generated potential-energy surfaces. A "spectroscopic" potential-energy surface of NH3 is determined by a slight empirical adjustment of the ab initio potential to the experimental vibrational term values. Variational calculations on this refined surface yield rms deviations from experiment of 0.8 cm-1 for 24 inversion splittings and 0.4 (3.0) cm-1 for 34 (51) vibrational term values up to 6100 (10,300) cm-1.     

Cold and ultracold NH-NH collisions: the field-free case

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X (3)Σ(-)) + NH(X (3)Σ(-)) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic (14)NH and bosonic (15)NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.     

Quasiclassical trajectory study of energy transfer and collision-induced dissociation in hyperthermal Ar + CH4 and Ar + CF4 collisions

We present a study of energy transfer in collisions of Ar with methane and perfluoromethane at hyperthermal energies (E(coll) = 4-10 eV). Quasiclassical trajectory calculations of Ar + CX(4) (X = H, F) collisions indicate that energy transfer from reagents' translation to internal modes of the alkane molecule is greatly enhanced by fluorination. The reasons for the enhancement of energy transfer upon fluorination are shown to emerge from a decrease in the hydrocarbon vibrational frequencies of the CX(4) molecule with increasing the mass of the X atom, and to an increase of the steepness of the Ar-CX(4) intermolecular potential. At high collision energies, we find that the cross section of Ar + CF(4) collisions in which the amount of energy transfer is larger than needed to break a C-F bond is at least 1 order of magnitude larger than the cross sections of Ar + CH(4) collisions producing CH(4) with energy above the dissociation limit. In addition, collision-induced dissociation is detected in short time scales in the case of the fluorinated species at E(coll) = 10 eV. These results suggest that the cross section for degradation of fluorinated hydrocarbon polymers under the action of nonreactive hyperthermal gas-phase species might be significantly larger than that of hydrogenated hydrocarbon polymers. We also illustrate a practical way to derive intramolecular potential energy surfaces for bond-breaking collisions by improving semiempirical Hamiltonians based on grids of high-quality ab initio calculations.     

Atom-centered symmetry functions for constructing high-dimensional neural network potentials

Neural networks offer an unbiased and numerically very accurate approach to represent high-dimensional ab initio potential-energy surfaces. Once constructed, neural network potentials can provide the energies and forces many orders of magnitude faster than electronic structure calculations, and thus enable molecular dynamics simulations of large systems. However, Cartesian coordinates are not a good choice to represent the atomic positions, and a transformation to symmetry functions is required. Using simple benchmark systems, the properties of several types of symmetry functions suitable for the construction of high-dimensional neural network potential-energy surfaces are discussed in detail. The symmetry functions are general and can be applied to all types of systems such as molecules, crystalline and amorphous solids, and liquids.     

He-HF vibrational inelasticity at low and intermediate energies from quantum collision theory

The rotational-sudden approximation (IOSA) has been applied to He-HF scattering at intermediate energies by using an ab initio computed potential-energy surface which explicitly included the dependence on the internal molecular coordinate. The vibrational degree of freedom has therefore been treated exactly by solving the coupled radial equations that were obtained after expanding the target vibrations over a large number of HO wavefunctions. The radial behaviour of the computed coupling matrix elements between lower-lying target states is analyzed at various relative orientations of the atom-to-target vector with respect to the molecular bond. The present results clearly show that the coupling strength and range can be directly related to the specific anisotropic behaviour of the potential surface and to the varying efficiency of (V, T) coupling as the He atom probes different molecular regions. The computed relaxation times are compared with experiments at various temperatures and with previous calculations performed via different surfaces and dynamical models and show satisfactory agreement with observations, thus markedly improving the quality of earlier computed cross sections.     

Theoretical study of the Ar-, Kr-, and Xe-CH4, -CF4 intermolecular potential-energy surfaces

We present a theoretical study of the intermolecular potentials for the Ar, Kr, and Xe-CH4, -CF4 systems. The potential-energy surfaces of these systems have been calculated utilizing second-order M?ller-Plesset perturbation theory and coupled-cluster theory in combination with correlation-consistent basis sets (aug-cc-pvnz; n = d, t, q). The calculations show that the stabilizing interactions between the rare gases and the molecules are slightly larger for CF4 than for CH4. Moreover, the rare-gas-CX4 (X = H, F) potentials are more attractive for Xe than for Kr and Ar. Our highest quality ab initio data (focal-point-CCSD(T) extrapolated to the complete basis set limit) have been used to develop pairwise analytical potentials for rare-gas-hydrocarbon (-fluorocarbon) systems. These potentials can be applied in classical-trajectory studies of rare gases interacting with hydrocarbon surfaces.     

Experimental and theoretical study of the infrared spectra of BrHI- and BrDI-

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated.     

Direct calculation of coupled diabatic potential-energy surfaces for ammonia and mapping of a four-dimensional conical intersection seam

We used multiconfiguration quasidegenerate perturbation theory and the fourfold-way direct diabatization scheme to calculate ab initio potential-energy surfaces at 3600 nuclear geometries of NH3. The calculations yield the adiabatic and diabatic potential-energy surfaces for the ground and first electronically excited singlet states and also the diabatic coupling surfaces. The diabatic surfaces and coupling were fitted analytically to functional forms to obtain a permutationally invariant 2 x 2 diabatic potential-energy matrix. An analytic representation of the adiabatic potential-energy surfaces is then obtained by diagonalizing the diabatic potential-energy matrix. The analytic representation of the surfaces gives an analytic representation of the four-dimensional conical intersection seam which is discussed in detail.     

Spectroscopy of Ar-SH and Ar-SD. II. Determination of the three-dimensional intermolecular potential-energy surface

All the pure rotational transitions reported in the previous studies [J. Chem. Phys. 113, 10121 (2000); J. Mol. Spectrosc. 222, 22 (2003)] and newly observed rotation-vibration transitions, P = 1/2 <-- 3/2, for Ar-SH and Ar-SD [J. Chem. Phys. (2005), the preceding paper] have been simultaneously analyzed to determine a new intermolecular potential-energy surface of Ar-SH in the ground state. A Schrodinger equation considering the three-dimensional freedom of motion for an atom-diatom complex in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational levels using the discrete variable representation method. A three-dimensional potential-energy surface is determined by a least-squares fitting with initial values of the parameters for the potential obtained by ab initio calculations at the RCCSD(T)/aug-cc-pVTZ level of theory. The potential well reproduces all the observed data in the microwave and millimeter wave regions with parity doublings and hyperfine splittings. Several low-lying rovibrational energies are calculated using the new potential-energy surface. The dependence of the interaction energy between Ar and SH(2pi(i)) on the bond length of the SH monomer is discussed.     

Ab initio vibrational predissociation dynamics of He-I2(B) complex

Three-dimensional quantum mechanical calculations on the vibrational predissociation dynamics of HeI2 B state complex are performed using a potential energy surface accurately fitted to unrestricted open-shell coupled cluster ab initio data, further enabling extrapolation for large I2 bond lengths. A Lanczos iterative method with an optimized complex absorbing potential is used to determine energies and lifetimes of the vibrationally predissociating He,I2(B,v') complex for v'相似文献   

Microwave spectra of the Xe-N2 van der Waals complex: a comparison of experiment and theory

Rotational transitions for the Xe-N2 complex were measured in the frequency region from 4 to 18 GHz using a pulsed-nozzle Fourier-transform microwave spectrometer. Twelve (four) a-type transitions were recorded for the 132Xe-14N2 and 129Xe-14N2 (131Xe-15N)) isotopomers. In addition, the nuclear quadrupole hyperfine structures due to the presence of the 14N (nuclear-spin quantum number I=1) and 131Xe (I=32) nuclei were detected and analyzed. Two ab initio potential-energy surfaces were calculated at the coupled-cluster level of theory with single, double, and pertubatively included triple excitations. Dunning's augmented correlation-consistent polarized valence triple-zeta basis set was used for the nitrogen atoms. For the first surface, a well-tempered basis set with additional polarization functions was used for the Xe atom; for the second surface, a newly developed augmented correlation-consistent polarized valence quintuple-zeta basis set employing small-core relativistic pseudopotentials was used for the Xe atom. The basis sets were supplemented with bond functions for the van der Waals bond. The counterpoise correction was applied to reduce the basis-set superposition error. The resulting two surfaces both have a single minimum at a T-shaped geometry, with well depths of 122.4 and 119.3 cm(-1), respectively. Bound-state energies supported by the potential-energy surface were determined. The quality of the ab initio potential-energy surfaces was evaluated by comparison of the experimental transition frequencies and rotational and centrifugal distortion constants with those derived from the bound-state energies. A scaled potential-energy surface was obtained which has excellent agreement with the experimental data.     

Analytical ab initio potential-energy surfaces for the ground and the first singlet excited states of HeH2

Analytical potential-energy surfaces have been constructed for the ground and the first excited states of HeH₂. The functions fit ab initio MRD CI calculations with standard deviations of 0.05 and 0.13 eV for the ground and the excited surface respectively. Classical trajectory calculations for collisions of ⁴Hc with HD(B ¹Σ_u⁺, υ = 3, J = 2) at the temperature T = 297 K yields the electronic quenching cross section σ_Q = 6.5 A² and the vibrational cross section σ_3→2 = 3.8 A². The results are in qualitative agreement with the experimental values of Fink, Akins and Moore.     

Interaction energies in hydrogen-bonded systems: a testing ground for subsystem formulation of density-functional theory

The formalism based on the total energy bifunctional (E[rhoI,rhoII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN-HF, H2CO-H2O, and MeOH-H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[rhoI,rhoII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO-H2O and H2O-H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 A) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.     

On the (Emultiply sign in circlee)-Jahn-Teller conical intersections in the 3p(E') and 3d(E") Rydberg electronic states of triatomic hydrogen

The static and dynamic aspects of the Jahn-Teller (JT) interactions in the 3p(E') and 3d(E") Rydberg electronic states of H3 are analyzed theoretically. The static aspects are discussed based on recent ab initio quantum chemistry results, and the dynamic aspects are examined in terms of the vibronic spectra and nonradiative decay behavior of these states. The adiabatic potential-energy surfaces of these degenerate electronic states are derived from extensive ab initio calculations. The calculated adiabatic potential-energy surfaces are diabatized following our earlier study on this system in its 2p(E') ground electronic state. The nuclear dynamics on the resulting conically intersecting manifold of electronic states is studied by a time-dependent wave-packet approach. Calculations are performed both for the uncoupled and coupled state situations in order to understand the importance of nonadiabatic interactions due to the JT conical intersections in these excited Rydberg electronic states.     

A wave-packet simulation of the low-lying singlet electronic transitions of acetylene

The vibronic structure of the S0 --> S1 and the S0 --> S2 electronic transitions of acetylene is studied theoretically based on an ab initio quantum-dynamical approach. The underlying potential-energy surfaces and transition dipole moment functions are obtained from high-level multireference calculations, including the Davidson correction. Ensuing quantum-dynamical simulations rely on the wave-packet propagation method, using grid techniques, and including three nuclear degrees of freedom (C-C stretching and both HCC bending modes for J = 0). The importance of strong anharmonicity is assessed, especially for the S2 excited state with its unusual potential-energy surface. Good overall agreement with the experimental UV absorption spectrum of acetylene is achieved in the range of 6-8 eV.     

Ab initio and direct quasiclassical-trajectory study of the F+CH4-->HF+CH3 reaction

We present an electronic structure and dynamics study of the F+CH4-->HF+CH3 reaction. CCSD(T)/aug-cc-pVDZ geometry optimizations, harmonic-frequency, and energy calculations indicate that the potential-energy surface is remarkably isotropic near the transition state. In addition, while the saddle-point F-H-C angle is 180 degrees using MP2 methods, CCSD(T) geometry optimizations predict a bent transition state, with a 153 degrees F-H-C angle. We use these high-quality ab initio data to reparametrize the parameter-model 3 (PM3) semiempirical Hamiltonian so that calculations with the improved Hamiltonian and employing restricted open-shell wave functions agree with the higher accuracy data. Using this specific-reaction-parameter PM3 semiempirical Hamiltonian (SRP-PM3), we investigate the reaction dynamics by propagating quasiclassical trajectories. The results of our calculations using the SRP-PM3 Hamiltonian are compared with experiments and with the estimates of two recently reported potential-energy surfaces. The trajectory calculations using the SRP-PM3 Hamiltonian reproduce quantitatively the measured HF vibrational distributions. The calculations also agree with the experimental HF rotational distributions and capture the essential features of the excitation function. The results of the SRP semiempirical Hamiltonian developed here clearly improve over those using the two prior potential-energy surfaces and suggest that reparametrization of semiempirical Hamiltonians is a promising strategy to develop accurate potential-energy surfaces for reaction dynamics studies of polyatomic systems.     

Torsional eigenvalues of the water trimer on several ab initio potential surfaces

Eigenvalues corresponding to the three torsional degrees of freedom were calculated for the water trimer and its deuterated isotopomer in four sets of calculations involving different potential energy surfaces. The four potential surfaces were developed in this work by reparametrization of the CKL function against four sets of ab initio energies calculated with and without counterpoise correction. Transition frequencies corresponding to the low-frequency torsional motions of the trimer were calculated and then compared with those found from experiment to assess the accuracy of each potential energy surface. Although reparametrization of the CKL function to a set of counterpoise-corrected energies yielded transition energies that are in qualitative agreement with those from experiment, reparametrization to another set of counterpoise-corrected energies resulted in highly inaccurate values of the transition energy. As a consequence, our results demonstrate that caution must be exercised in the implementation of the counterpoise method as it does not always lead to more accurate ab initio calculations. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 233–252, 1998