Stabilised Coupled Trajectory Mixed Quantum Classical Algorithm with Improved Energy Conservation: CTMQC-EDI

TL;DR

This paper introduces CTMQC-EDI, an improved non-adiabatic dynamics algorithm that enhances energy conservation and stability by redefining the quantum momentum, making it suitable for large molecular systems.

Contribution

The paper presents a novel modification to CTMQC-E, called CTMQC-EDI, which eliminates divergences in quantum momentum and improves energy conservation in non-adiabatic dynamics.

Findings

Significantly improved energy conservation in Tully models I-IV.

Negligible spurious population transfer in strong coupling regions.

Enhanced numerical stability and scalability for large systems.

Abstract

Coupled trajectory mixed quantum classical (CTMQC) dynamics is a rigorous approach to trajectory-based non-adiabatic dynamics, which has recently seen an improvement to energy conservation via the introduction of the CTMQC-E algorithm. Despite this, the method's two key quantities distinguishing it from Ehrenfest dynamics, the modified Born-Oppenheimer momentum and the quantum momentum, require regularisation procedures in certain circumstances. Such procedures in the latter can cause instabilities leading to undesirable effects such as energy drift and spurious population transfer, which is expected to become increasingly prevalent the larger the system as such events would happen more frequently. We propose a further modification to CTMQC-E which includes a redefinition of the quantum momentum, CTMQC-EDI (Double Intercept), such that it has no formal divergences. We then show for Tully models I-IV that the algorithm has greatly improved total energy conservation and negligible spurious population transfer at all times, in particular in regions of strong non-adiabatic coupling. CTMQC-EDI therefore shows promise as a numerically robust non-adiabatic dynamics technique that accounts for decoherence from first principles and that is scalable to large molecular systems and materials.