Towards Accurate Mixed Quantum Classical Simulations of Vibrational Polaritonic Chemistry

TL;DR

This paper advances mixed quantum-classical simulations for vibrational polaritonic chemistry by introducing the MASH method with a quantum cavity mode, improving accuracy and scalability for collective molecular systems.

Contribution

It introduces the mapping approach to surface hopping (MASH) with a quantum cavity mode, addressing limitations of traditional MQC methods in vibrational polaritonic chemistry.

Findings

Combining MASH with a quantum cavity mode yields the most accurate reaction rates.

The $ ext{ extbackslash epsilon}$-MASH approach addresses size-inconsistency issues.

Quantum treatment of the cavity mode affects long-time population dynamics.

Abstract

Interest in vibrational polaritonic chemistry, where ground-state chemical kinetics are modified via confined optical modes in a cavity, has surged in recent years. Although models have been developed to understand cavity-modified reactions, fully quantum mechanical simulations remain out of reach for the collective regime that involves many molecules, a critical aspect of the phenomenon. Mixed quantum-classical (MQC) simulations offer a scalable alternative, but their accuracy requires testing and potential improvements even in the single-molecule limit. In this work, we take this step by first introducing the mapping approach to surface hopping (MASH) to address the limitations of traditional MQC methods. Second, we incorporate a quantum treatment of the cavity mode, moving beyond the classical approximations often employed in previous studies. Results for a single-molecule model of vibrational polaritonic chemistry show that combining MASH with a quantum cavity mode yields the most accurate rates. However, this scheme may produce different long-time population dynamics at zero coupling depending on whether the cavity mode is quantized; a problem known as size-inconsistency in MASH. We address this problem proposing the $\epsilon$-MASH approach, which forbids hopping between states with negligible nonadiabatic couplings (NACs). Combining MASH with a quantum cavity mode thus provides a promising approach for scalable and accurate MQC simulations in the collective regime.