A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol^?1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m_A = 20 u, m_B = 1 u, m_C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r_BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.     

Reactive bands and rebounding trajectories in collinear F + H2

Bands of reactive and unreactive trajectories have been mapped for classical collinear F + H₂ (υ = 0). The edges of the band are characterized by trajectories which undergo several reflections in the corner of the potential energy surface. Multiple reflections are seen to lead to less sharply defined band edges.     

Flip rearrangement in the water pentamer: Analysis of electronic structure

Tunneling pathways of the flip rearrangement between permutation-inversion isomers corresponding to the energetically degenerate global energy minima of (H₂O)₅ are analyzed in terms of the electronic structure. We demonstrate that charge density-based scalar measures quantify the responses of the bonding to the flip rearrangement and we discovered a high degree of continuity of the values that depend on the presence of the sliding motion of the bond critical point relative to the oxygen atom. The scalar measures can distinguish the pairs of permutation-inversion isomers everywhere except at the transition state due to the asymmetrical energy barrier; however, they cannot determine the most and least facile directions of the flip rearrangement. The vector or directional character of the two sides of the pathway is captured by the stress tensor trajectories constructed in a non-Cartesian space, defined by the variation of the position of the bond critical point. The stress tensor trajectories are presented in terms that enable bond-flexing, bond-twist, and bond-anharmonicity of the flip rearrangement between permutation-inversion isomers to be quantified. The stress tensor trajectories can distinguish the isomers at the transition state and demonstrate that the clockwise and counter-clockwise directions of the flip rearrangement are the most and least facile respectively.     

Dynamical effects of mode specific excitation in unimolecular decomposition: A trajectory study of C2H6

Classical trajectories on a well coupled model potential energy surface have been used to study the effects of mode localized excitation on unimolecular decomposition in C₂H₆. The results display both apparent and intrinsic non-statistical effects that can be ascribed to restrictions in intramolecular energy transfer both among the CH vibrations and between these CC motion.     

Quasiclassical trajectory state-to-state cross sections for energy transfer in Ar+ OH(υ = 9,J = 0,4, and 8) collisions

State-to-state energy transfer cross sections have been computed for Ar + OH(υ₁ = 9; J_i = 0,4, and 8) at initial relative translational energy 0.2 eV using quasiclassical trajectories. The results show that a relatively small amount of initial rotational excitation has a significant effect on the energy transfer. The energy transfer for J_i = 8 is dominated by transitions for which the vibrational and rotational energy changes are such that the translational energy changes are minimized.     

Vectorization of the general Monte Carlo classical trajectory program VENUS

The general chemical dynamics computer program VENUS is used to perform classical trajectory simulations for large polyatomic systems, with many atoms and complicated potential energy functions. To simulate an ensemble of many trajectories requires a large amount of CPU time. Since each trajectory is independent, it is possible to parallel process a large set of trajectories instead of processing the trajectories by the conventional sequential approach. This enhances the vectorizability of the VENUS program, since the integration of Hamilton's equations of motion and the gradient evaluation, which comprise 97.8% of the CPU, can each be parallel processed. In this article, the vectorization and ensuing optimization of VENUS on the CRAY-YMP and IBM-3090 are presented in terms of both global strategies and technical details. A switching algorithm is designed to enhance the vector performance and to minimize the memory storage. A performance of 140 MFLOPS and a vector/scalar execution rate ratio of 10.6 are observed when this new version of VENUS is used to study the association of CH₃ with the H(Ar)₁₂ cluster on the CRAY-YMP.     

Ab-initio molecular dynamics based on non-local density functional procedure with Gaussian basis; study of structural and temperature behaviour of metallic clusters

An ab-initio molecular dynamics procedure without precalculation of the Born-Oppenheimer energy surface based on an iterative non-local density functional method employing Gaussian atomic basis has been developed. Analytical gradients are calculated and used for the propagation of nuclei. Sufficiently long trajectories can be calculated at an acceptable computational cost, allowing for analysis of dynamical behaviour of small metallic clusters. This is illustrated on an example of the Li₈ cluster. Temperature behaviour of different type of isomers has been investigated. Calculated power spectra allow to identify the presence of more than one isomeric forms along the given trajectories.     

A quasiclassical trajectory study of the collisional dissociation of H2 by H atoms

The collision induced dissociation of H₂ by H atoms was studied by quasiclassical trajectories using the Liu-Siegbahn-Truhlar-Horowitz potential energy surface. Dissociation cross sections were obtained for five highly internally excited initial states of H₂ for translational energies up to 100 kcal mol^?1. Rate constants for dissociation out of these states were calculated for temperatures of 1000 to 10,000 K. Initial internal energy strongly enhances the probability of collisional dissociation, vibrational energy being more effective than rotational. The results are compared to those from a similar study of the H₂?He system, and are discussed in relation tothe respective potential energy surfaces. The implications for the kineticsof thermal dissociation are also considered.     

Quasi-classical trajectories study of the chemisorption of H2 on the Pt(111) surface: effect of surface motion

The effect of energy exchange on the dynamics of the chemisorption of H₂ on Pt (111) is studied within a LEPS potential quasi-classical trajectories treatment. The Einstein model is assumed for the solid. The rigidity of the surfaces is shown to be an acceptable approximation.     

Quasiperiodic motion in collinear HeH2+

The classical motion of the highly vibrationally excited HeH₂⁺ collinear complex has been investigated. An estimated of the fraction of phase containing quasiperiodic trajectories versus total energy is reported. At some energies it is found that dissociative and quasiperiodic trajectories occupy disjoint regions of phase space.     

Differential Cross Sections for the F + H2 Reaction: A Comparison of Classical Trajectory Results for Two Potential Energy Surfaces

Classical trajectories were calculated for the F + H₂ reaction over two potential energy surfaces: (1) the well known Muckerman 5 surface (Theoretical Chemistry: Advances and Perspectives 1981 , 6A, 1) and (2) the recent surface of Stark and Werner (J. Chem. Phys. 1996 , 104, 6515). Integral cross sections, state specified cross sections, differential cross sections and product energy distributions were calculated for the two surfaces. Since the methods for calculating the trajectories and expressing differential cross sections were identical for both surfaces, the rather substantial differences in the results are clearly due to differences in the potential surfaces. The results are discussed in terms of the special characteristics of the two surfaces.     

Bounday trajectories in collision-induced dissociation

The boundary trajectories separating non-reactive, reactive, and dissociative bands of trajectories in atom—diatom collision-induced dissociation are defined. Trajectory calculations are reported for the collinear H + H₂ reaction which demonstrate how the boundary trajectories can be used to obtained information on the non-reactive, reactive, and dissociative regions of the reagents phase space.     

Ab initio classical trajectory calculations of 1,3-cyclobutanedione radical cation dissociation

Abstract  

The dissociation of 1,3-cyclobutanedione radical cation was studied by ab initio direct classical trajectory calculations at the BH&HLYP/6-31G(d) level of theory. A microcanonical ensemble using normal mode sampling was constructed by distributing 10 kcal/mol of excess energy above the transition state for the tautomerization of the keto-enol to the diketo form. A total of 210 trajectories were run starting from this transition state, yielding chemically activated 1,3-cyclobutanedione radical cation. The majority of the trajectories resulted in CH₂CO^+· + CH₂CO, with the activated CC bond breaking nearly twice as often as the spectator CC bond. The non-statistical behavior is observed because the rate of energy redistribution within the molecule is comparable to or slower than the dissociation rates. In addition to the expected products, dissociation to CH₂COCH₂^+· + CO and formation of a proton-transferred product HCCO^· + CH₃CO⁺ were also seen in some of the trajectories.     

Structure reorganization dynamics of water clusters upon vertical ionization: Quantum chemical investigation

Quantum dynamics simulations of (H₂O) _n ⁺ water cluster cations comprising up to six molecules with the initial configurations of stable neutral isomers are combined with stationary calculations of the same cations in order to get nonempirical information about water cluster relaxation after the absorption of energy in a range of from 11.0 to 12.0 eV. The electronic problem was solved and the potential energy of the system was estimated in terms of the restricted or unrestricted Hartree-Fock approximation with the 6–31++G** atomic basis set taking into account second-order Möller-Plesset perturbation theory corrections. Calculations of dynamic trajectories were based on the Born-Oppenheimer approximation. The formation of stable water cluster cations is shown to be a multistage process, the principal stages of which are qualitatively correctly reflected by the sequence of steps in cation geometry optimization runs.     

O–O2 state-to-state vibrational relaxation and dissociation rates based on quasiclassical calculations

A complete set of V–T (vibration–translation) relaxation rates and of dissociation coefficients for the system O–O₂ have been obtained by using quasiclassical trajectories on the Varandas and Pais potential energy surface. The results, averaged on a Boltzmann rotational distribution, cover the whole range of the vibrational ladder and are reproduced in closed form ready to be implemented in state-to-state kinetic models. The accuracy of the results has been tested by comparing them with available experimental and theoretical values (ASI-CAST project is acknowledged).     

Impact of the exchange-correlation functional on the structure of glassy GeSe2

The structural properties of glassy GeSe₂ were studied by using first-principles molecular dynamics with the Becke, Lee, Yang and Parr (BLYP) expression for the exchange-correlation energy within density functional theory. A comparison is made with the results previously obtained for this material by using first-principles molecular dynamics with the Perdew and Wang (PW) exchange-correlation functional. Overall, the structures of the BLYP-GeSe₂ and PW-GeSe₂ networks are quite similar, the BLYP approach favoring a larger number of Ge–Ge homopolar bonds, in better agreement with the experimental results. The BLYP network does, however, feature a smaller fraction of corner-sharing motifs by comparison with the PW network but the fraction of edge-sharing motifs is the same for both structures, at least within the confines of an approach based on a single temporal trajectory. Further studies are required to determine whether agreement between the BLYP structure and experiment can be improved by taking the average over a larger number of temporal trajectories or whether additional developments are required for the exchange-correlation part of the energy functional.     

Spectral Diffusion of Single Molecules in a Hierarchical Energy Landscape

Spectral diffusion as a result of both the transitions between different molecular conformers and the ′′molecular softness′′ of quasi‐free perylene diimides on a SiO₂ surface is investigated by means of single‐molecule spectroscopy, which reveals the time dependence of both the fluorescence spectra and the three‐dimensional orientation. Spectral wavelengths of all single emitters cover a wide energy range of about 0.27 eV, which is due to different types of conformers with large differences in optical transition energy. Time‐dependent spectral trajectories of single emitters within this wavelength manifold are evaluated with a model transcribed from the analysis of spatial diffusion. Spectral diffusion processes are closely correlated with fluorescence emission and excitation power. The overall analysis of spectral diffusion reveals, similar to proteins, a hierarchy of energy barriers in a broad energy landscape.     

On the rate and activation energy of the Br + Br2 ataom-exchange reaction

The Br + Br₂ atom-exchange reaction has been studied using quasiclassical trajectories and a semi-empirical potential-energy surface with shallow, long-range wells. The computed reaction rate coefficient is in good agreement with experiment. The computed reaction activation energy is ? 1 kcal/mole.