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Journal ArticleDOI

Comparisons of classical chemical dynamics simulations of the unimolecular decomposition of classical and quantum microcanonical ensembles.

14 May 2012-Journal of Chemical Physics (American Institute of Physics)-Vol. 136, Iss: 18, pp 184110-184110
TL;DR: The relevance and accuracy of the classical trajectory simulation of the quantum microcanonical ensemble, for obtaining the quantum anharmonic RRKM rate constant, is discussed.
Abstract: Previous studies have shown that classical trajectory simulations often give accurate results for short-time intramolecular and unimolecular dynamics, particularly for initial non-random energy distributions. To obtain such agreement between experiment and simulation, the appropriate distributions must be sampled to choose initial coordinates and momenta for the ensemble of trajectories. If a molecule's classical phase space is sampled randomly, its initial decomposition will give the classical anharmonic microcanonical (RRKM) unimolecular rate constant for its decomposition. For the work presented here, classical trajectory simulations of the unimolecular decomposition of quantum and classical microcanonical ensembles, at the same fixed total energy, are compared. In contrast to the classical microcanonical ensemble, the quantum microcanonical ensemble does not sample the phase space randomly. The simulations were performed for CH4, C2H5, and Cl−‐‐‐CH3Br using both analytic potential energy surfaces and direct dynamics methods. Previous studies identified intrinsic RRKM dynamics for CH4 and C2H5, but intrinsic non-RRKM dynamics for Cl−‐‐‐CH3Br. Rate constants calculated from trajectories obtained by the time propagation of the classical and quantum microcanonical ensembles are compared with the corresponding harmonic RRKM estimates to obtain anharmonic corrections to the RRKM rate constants. The relevance and accuracy of the classical trajectory simulation of the quantum microcanonical ensemble, for obtaining the quantum anharmonic RRKM rate constant, is discussed.
Citations
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Journal ArticleDOI
TL;DR: The VENUS/NWChem interface is designed to link the general electronic structure program (N WChem) and classical chemical dynamics simulation program (VENUS) to perform direct dynamics simulation in which the trajectories “on the fly” with the potential and its derivatives obtained directly from electronic structure theory.

90 citations

Journal ArticleDOI
TL;DR: A dynamically biased statistical model is presented to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT), and the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough.
Abstract: In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)10.1063/1.2430711]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H5+ complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calcu...

35 citations

Journal ArticleDOI
TL;DR: The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.
Abstract: A zero-point energy (ZPE) constraint model is proposed for classical trajectory simulations of unimolecular decomposition and applied to CH4* → H + CH3 decomposition. With this model trajectories are not allowed to dissociate unless they have ZPE in the CH3 product. If not, they are returned to the CH4* region of phase space and, if necessary, given additional opportunities to dissociate with ZPE. The lifetime for dissociation of an individual trajectory is the time it takes to dissociate with ZPE in CH3, including multiple possible returns to CH4*. With this ZPE constraint the dissociation of CH4* is exponential in time as expected for intrinsic RRKM dynamics and the resulting rate constant is in good agreement with the harmonic quantum value of RRKM theory. In contrast, a model that discards trajectories without ZPE in the reaction products gives a CH4* → H + CH3 rate constant that agrees with the classical and not quantum RRKM value. The rate constant for the purely classical simulation indicates that anharmonicity may be important and the rate constant from the ZPE constrained classical trajectory simulation may not represent the complete anharmonicity of the RRKM quantum dynamics. The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.

32 citations

Journal ArticleDOI
TL;DR: Simulations of the dissociation of microcanonical ensembles of Bz2 vibrational states, at energies E corresponding to temperatures T of 700-1500 K, were simulated to study intramolecular vibrational energy redistribution (IVR) between the intramolescular and intermolecular modes.
Abstract: Classical chemical dynamics simulations were performed to study the intramolecular and unimolecular dissociation dynamics of the benzene dimer, Bz2 → 2 Bz. The dissociation of microcanonical ensembles of Bz2 vibrational states, at energies E corresponding to temperatures T of 700–1500 K, were simulated. For the large Bz2 energies and large number of Bz2 vibrational degrees of freedom, s, the classical microcanonical (RRKM) and canonical (TST) rate constant expressions become identical. The dissociation rate constant for each T is determined from the initial rate dN(t)/dt of Bz2 dissociation, and the k(T) are well-represented by the Arrhenius eq k(T) = A exp(−Ea/RT). The Ea of 2.02 kcal/mol agrees well with the Bz2 dissociation energy of 2.32 kcal/mol, and the A-factor of 2.43 × 1012 s–1 is of the expected order-of-magnitude. The form of N(t) is nonexponential, resulting from weak coupling between the Bz2 intramolecular and intermolecular modes. With this weak coupling, large Bz2 vibrational excitation, an...

22 citations

Journal ArticleDOI
TL;DR: A detailed picture is presented of the [CH3--I--OH](-) dissociation dynamics and is very important for unraveling the role of [CH2I-- OH](-) in the dynamics of the OH(-)(H2O)(n=1,2) + CH3I reactions.
Abstract: Direct dynamics simulations were used to study dissociation of the [CH3--I--OH]− complex ion, which was observed in a previous study of the OH– + CH3I gas phase reaction (J. Phys. Chem. A 2013, 117, 7162). Restricted B97-1 simulations were performed to study dissociation at 65, 75, and 100 kcal/mol and the [CH3--I--OH]− ion dissociated exponentially, in accord with RRKM theory. For these energies the major dissociation products are CH3I + OH–, CH2I– + H2O, and CH3OH + I–. Unrestricted B97-1 and restricted and unrestricted CAM-B3LYP simulations were also performed at 100 kcal/mol to compare with the restricted B97-1 results. The {CH3I + OH–}:{CH2I– + H2O}:{CH3OH + I–} product ratio is 0.72:0.15:0.13, 0.81:0.05:0.14, 0.71:0.19:0.10, and 0.83:0.13:0.04 for the restricted B97-1, unrestricted B97-1, restricted CAM-B3LYP, and unrestricted CAM-B3LYP simulations, respectively. Other product channels found are CH2 + I– + H2O, CH2 + I–(H2O), CH4 + IO–, CH3– + IOH, and CH3 + IOH–. The CH3– + IOH singlet products are...

15 citations

References
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Journal ArticleDOI
TL;DR: An overview of NWChem is provided focusing primarily on the core theoretical modules provided by the code and their parallel performance, as well as Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures.

4,666 citations

Journal ArticleDOI
TL;DR: In this paper, a wave packet is decomposed into time-dependent wave packets, which spread minimally and which execute classical or nearly classical trajectories, assuming a Gaussian form for the wave packets and equations of motion for the Gaussians.
Abstract: In this paper we develop a new approach to semiclassical dynamics which exploits the fact that extended wavefunctions for heavy particles (or particles in harmonic potentials) may be decomposed into time−dependent wave packets, which spread minimally and which execute classical or nearly classical trajectories. A Gaussian form for the wave packets is assumed and equations of motion are derived for the parameters characterizing the Gaussians. If the potential (which may be nonseparable in many coordinates) is expanded in a Taylor series about the instantaneous center of the (many−particle) wave packet, and up to quadratic terms are kept, we find the classical parameters of the wave packet (positions, momenta) obey Hamilton’s equation of motion. Quantum parameters (wave packet spread, phase factor, correlation terms, etc.) obey similar first order quantum equations. The center of the wave packet is shown to acquire a phase equal to the action integral along the classical path. State−specific quantum information is obtained from the wave packet trajectories by use of the superposition principle and projection techniques. Successful numerical application is made to the collinear He + H2 system widely used as a test case. Classically forbidden transitions are accounted for and obtained in the same manner as the classically allowed transitions; turning points present no difficulties and flux is very nearly conserved.

1,402 citations

Journal ArticleDOI
TL;DR: In this paper, the authors derived the classical Hamiltonian of a polyatomic system in terms of these coordinates and their conjugate momenta for the general case of an N atom system with a given nonzero value of the total angular momentum.
Abstract: The reaction path on the potential energy surface of a polyatomic molecule is the steepest descent path (if mass‐weighted Cartesian coordinates are used) connecting saddle points and minima. For an N‐atom system in 3d space it is shown how the 3N‐6 internal coordinates can be chosen to be the reaction coordinate s, the arc length along the reaction path, plus (3N‐7) normal coordinates that describe vibrations orthogonal to the reaction path. The classical (and quantum) Hamiltonian is derived in terms of these coordinates and their conjugate momenta for the general case of an N atom system with a given nonzero value of the total angular momentum. One of the important facts that makes this analysis feasible (and therefore interesting) is that all the quantities necessary to construct this Hamiltonian, and thus permit dynamical studies, are obtainable from a relatively modest number of ab initio quantum chemistry calculations of the potential energy surface. As a simple example, it is shown how the effects o...

1,296 citations

Journal ArticleDOI
TL;DR: In this paper, a new and convenient semiclassical method is proposed for the model molecular vibrational spectra investigated in this paper, which relies only upon classical trajectories and Gaussian integrals.
Abstract: A new and convenient semiclassical method is proposed. It relies only upon classical trajectories and Gaussian integrals. It seems to work very well for the model molecular vibrational spectra investigated here. It should be applicable to a wide variety of processes and can be variationally improved if necessary.

689 citations