Equilibrium-nonequilibrium ring-polymer molecular dynamics for nonlinear spectroscopy

TL;DR

This paper introduces an equilibrium-nonequilibrium ring-polymer molecular dynamics method that efficiently incorporates nuclear quantum effects into nonlinear optical spectroscopy simulations, improving accuracy over classical approaches.

Contribution

The authors develop a new RPMD-based method that accurately accounts for quantum effects in nonlinear spectroscopy, reducing computational cost and complexity compared to existing techniques.

Findings

Demonstrates advantages over classical and DKT-based methods

Exact in the classical limit, reducing to classical MD

Enables application of real-time path-integral techniques

Abstract

Two-dimensional Raman and hybrid terahertz/Raman spectroscopic techniques provide invaluable insight into molecular structure and dynamics of condensed-phase systems. However, corroborating experimental results with theory is difficult due to the high computational cost of incorporating quantum-mechanical effects in the simulations. Here, we present the equilibrium-nonequilibrium ring-polymer molecular dynamics (RPMD), a practical computational method that can account for nuclear quantum effects on the two-time response function of nonlinear optical spectroscopy. Unlike a recently developed approach based on the double Kubo transformed (DKT) correlation function, our method is exact in the classical limit, where it reduces to the established equilibrium-nonequilibrium classical molecular dynamics method. Using benchmark model calculations, we demonstrate the advantages of the equilibrium-nonequilibrium RPMD over classical and DKT-based approaches. Importantly, its derivation, which is based on the nonequilibrium RPMD, obviates the need for identifying an appropriate Kubo transformed correlation function and paves the way for applying real-time path-integral techniques to multidimensional spectroscopy.