PyUNIxMD: A Python‐based excited state molecular dynamics package

Abstract

Theoretical/computational description of excited state molecular dynamics is nowadays a crucial tool for understanding light-matter interactions in many materials. Herewe present an open-source Python-based nonadiabatic molecular dynamics program package, namely PyUNIxMD, to deal with mixed quantum-classical dynamics for correlated electron-nuclear propagation. The PyUNIxMD provides many interfaces for quantum chemical calculation methods with commercial and noncommercial ab initio and semiempirical quantum chemistry programs. In addition, the PyUNIxMD offers many nonadiabatic molecular dynamics algorithms such as fewest-switch surface hopping and its derivatives as well as decoherence-induced surface hopping based on the exact factorization (DISH-XF) and coupled-trajectory mixed quantum-classical dynamics (CTMQC) for general purposes. Detailed structures and flows of PyUNIxMD are explained for the further implementations by developers. We perform a nonadiabatic molecular dynamics simulation for a molecular motor system as a simple demonstration.

Exact-Factorization-Based Surface-Hopping for Multi-State Dynamics

“…This property is especially enticing for SH schemes, where the disconnect between the coherent electronic evolution propagating in a superposition of states and the nuclear trajectory evolving on a single state at any given time leads to the problem commonly known as "overcoherence". Thus the exact factorization-based surface-hopping scheme (SHXF) was introduced, where the electronic equation from CTMQC is used in the SH algorithm [18,19] and restores the internal consistency, incorporating decoherence from first-principles. This method has been demonstrated on a range of fascinating light-induced processes on complex molecules [20][21][22][23].…”

“…The original SH algorithm involves just the first two terms on the right in Eq. ( 1), while in SHXF [18,19], which uses the electronic equation coming from the mixed quantum-classical limit of the exact factorization approach, we have…”

“…[18] for details on the formalism and algorithm. Along with other MQC approaches for correlated electron-nuclear dynamics simulations, SHXF is implemented in the molecular dynamics package, PyUNIxMD [19].…”

“…The SH, SHEDC and SHXF calculations are performed using the PyUNIxMD code [19], while A-FSSH computations are done with the SHARC 2.0 code [47][48][49]. The momentum is rescaled after a hop in the direction of the non-adiabatic coupling vector (NACV), and after a frustrated hop the momentum direction is preserved, unless otherwise stated.…”

“…The implementation of the SHXF algorithm of Ref. [18,19] through auxiliary trajectories launched for each independent trajectory is a computationally efficient approach that will likely lead to improved predictions of surface-hopping dynamics for complex photo-excited dynamics in molecules beyond two-state dynamics, and could also be used in conjunction with a quantum trajectory SH scheme where ad hoc velocity adjustment is not required [50]. It is also likely to be promising for situations where multiple non-adiabatic events occur, including laser-driven processes.…”