Implementation of quasiclassical mapping approaches for nonadiabatic molecular dynamics in the PySurf package†

Quasiclassical methods for nonadiabatic molecular dynamics, based on Mayer–Miller–Stock–Thoss mapping, are implemented in the open source computer package PySurf. This complements the implementation of surface hopping approaches performed in previous studies, and leads to a unified code that allows nonadiabatic dynamics simulations using various mapping approaches (Ehrenfest dynamics, the linearised semiclassical initial value representation, the Poisson-bracket mapping equation, the “unity” approach for the identity operator, the spin mapping, and the symmetrical quasiclassical windowing method) as well as different flavours of surface hopping (fewest-switches, Landau–Zener, and a mapping-inspired scheme). Furthermore, a plugin is developed to provide diabatic vibronic models as input in a sum-of-products form. This opens the way to the benchmark of different types of trajectory-based propagators on different models, against exact quantum dynamical simulations performed, e.g., by the multiconfigurational time-dependent Hartree method. Illustrative calculations, performed using the whole set of available propagators, are presented for different harmonic and anharmonic two-state models, exhibiting various degrees of correlation between vibrational modes.