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Quasiclassical sampling and Wigner sampling of initial vibrational coordinates and momenta for polyatomic molecules in Monte Carlo molecular dynamics simulations

Journal Article · · Molecular Physics
 [1];  [2];  [1]
  1. University of Minnesota, Minneapolis, MN (United States)
  2. Jackson State University, MS (United States)
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In a quasiclassical trajectory simulation, the vibrational modes are initialised with quantised vibrational energies, but vibrational phases are sampled by Monte Carlo. This requires an algorithm to assign coordinates and momenta to the various atoms. In this work, we present two methods for implementing this for nonrotating polyatomic molecules, namely, fixed-energy vibrational-state-selected initial conditions and thermal initial conditions. We also present a method for initiating classical trajectories with a ground-state Wigner distribution. These vibrational treatments are sufficient to initialise trajectories for unimolecular processes, and we also show how they can be applied to simulate bimolecular collision processes. The treatments of unimolecular and bimolecular collision processes are available in two Python codes called wigner_state_selected.py and bimolecular_collision.py, respectively, which will generate initial condition files that are recognisable by the SHARC and SHARC-MN computer programs for dynamics calculations. Both codes are available as standalone programs, as well as being included in SHARC-MN, and they will be included in future versions of SHARC. Here, the methods implemented in these codes are mostly also available in the ANT computer program, and those that are not available in ANT will be incorporated in future versions of ANT.
Research Organization:
University of Minnesota, Saint Paul, MN (United States)
Sponsoring Organization:
National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
SC0015997
OSTI ID:
3011604
Journal Information:
Molecular Physics, Journal Name: Molecular Physics Journal Issue: 19-20 Vol. 123; ISSN 1362-3028; ISSN 0026-8976
Publisher:
Informa UK LimitedCopyright Statement
Country of Publication:
United States
Language:
English

References (9)

Monte Carlo Methods book January 1986
Reactive Scattering Cross Sections III: Quasiclassical and Semiclassical Methods book January 1979
ANT 2023: A program for adiabatic and nonadiabatic trajectories journal March 2024
Development and use of quantum mechanical molecular models. 76. AM1: a new general purpose quantum mechanical molecular model journal June 1985
General discussion journal January 1967
Photodissociation of LiFH and NaFH van der Waals complexes: A semiclassical trajectory study journal November 2001
Comparisons of classical and Wigner sampling of transition state energy levels for quasiclassical trajectory chemical dynamics simulations journal July 2010
Wigner phase space method: Analysis for semiclassical applications journal August 1976
A Note on the Generation of Random Normal Deviates journal June 1958

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Related Subjects

Wigner distribution
classical mechanics
mass-weighted coordinates
normal modes
trajectories
vibrational analysis