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Classical and nonclassical effects in surface hopping methodology for simulating coupled electronic-nuclear dynamics.

Craig C Martens1

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We compared two surface hopping methods for molecular dynamics: Fewest Switches Surface Hopping (FSSH) and Quantum Trajectory Surface Hopping (QTSH). QTSH naturally conserves quantum-classical energy, unlike FSSH, showing improved accuracy in simulations.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Chemical Physics

Background:

  • Simulating molecular dynamics with electronic transitions is crucial for understanding chemical reactions.
  • Surface hopping methods are widely used approximations for quantum-classical dynamics.
  • Existing methods like Fewest Switches Surface Hopping (FSSH) have limitations in energy conservation.

Purpose of the Study:

  • To analyze and compare the quantum-classical behavior of the Fewest Switches Surface Hopping (FSSH) and Quantum Trajectory Surface Hopping (QTSH) methods.
  • To investigate the accuracy of QTSH in conserving energy during molecular dynamics simulations with electronic transitions.
  • To compare simulation results with exact quantum mechanical calculations and Wigner representation dynamics.

Main Methods:

  • Employed an ensemble of independent trajectories undergoing stochastic transitions between electronic surfaces.
  • Applied Fewest Switches Surface Hopping (FSSH) with momentum rescaling for energy conservation.
  • Utilized Quantum Trajectory Surface Hopping (QTSH) incorporating a quantum force for natural energy conservation.

Main Results:

  • QTSH demonstrated natural conservation of the full quantum-classical energy, unlike FSSH which rescales momentum.
  • Population transfer and energy budgets were analyzed for model systems, showing differences between FSSH and QTSH.
  • Phase space dynamics of trajectory ensembles were compared with quantum evolution in the Wigner representation.

Conclusions:

  • QTSH offers a more accurate approach to molecular dynamics simulations involving electronic transitions due to its inherent energy conservation.
  • The study provides a detailed comparison of FSSH and QTSH, highlighting the advantages of QTSH for quantum-classical dynamics.
  • Further investigation into QTSH dynamics in phase space offers insights into quantum effects in complex systems.