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Non-equilibrium dynamics from RPMD and CMD.

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This study compares ring-polymer molecular dynamics (RPMD) and centroid molecular dynamics (CMD) for calculating quantum time correlation functions (TCFs). Both methods show accuracy comparable to equilibrium calculations for non-equilibrium conditions.

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Area of Science:

  • Quantum chemistry
  • Theoretical chemistry
  • Computational physics

Background:

  • Accurate calculation of quantum time correlation functions (TCFs) is crucial for understanding chemical dynamics.
  • Path-integral molecular dynamics methods like RPMD and CMD are popular for approximating these TCFs.
  • Their performance under non-equilibrium conditions, particularly for excited states, requires thorough investigation.

Purpose of the Study:

  • To evaluate the accuracy of ring-polymer molecular dynamics (RPMD) and centroid molecular dynamics (CMD) for non-equilibrium quantum time correlation functions (TCFs).
  • To compare the performance of RPMD and CMD against Matsubara dynamics and numerically exact methods.
  • To assess the applicability of these methods to systems undergoing excited-state dynamics.

Main Methods:

  • Investigated non-equilibrium TCFs using ring-polymer molecular dynamics (RPMD) and centroid molecular dynamics (CMD).
  • Analyzed TCFs for sudden vertical excitation and initial momentum impulse scenarios.
  • Compared results to Matsubara dynamics and employed a Liouville space hierarchical equation of motion approach for validation.

Main Results:

  • RPMD and CMD yield accurate non-equilibrium TCFs for linear operators under high temperatures, harmonic potentials, and at the t=0 limit.
  • Both methods maintain a connection to Matsubara dynamics for non-equilibrium initial conditions.
  • Numerical tests show RPMD and CMD achieve accuracy comparable to equilibrium TCF calculations, even for excited-state proton transfer.

Conclusions:

  • RPMD and CMD are reliable methods for calculating non-equilibrium quantum TCFs, demonstrating accuracy comparable to equilibrium calculations.
  • The methods show good agreement with theoretical predictions and exact calculations, extending their utility to dynamic processes.
  • These findings support the use of RPMD and CMD for studying complex quantum dynamics in various chemical systems.