Improved torque estimator for condensed-phase quasicentroid molecular dynamics

TL;DR

This paper presents an improved torque estimator for quasicentroid molecular dynamics, enhancing spectral accuracy and computational efficiency in simulating condensed-phase systems like water and ice.

Contribution

The authors develop a new intermolecular torque estimator and modify the mass-scaling in QCMD, significantly improving spectral results and reducing computational cost.

Findings

Eliminates a 25 cm$^{-1}$ red shift in libration bands.

Increases OH stretch band intensity in liquid water.

Reduces errors in radial distribution functions.

Abstract

We describe improvements to the quasicentroid molecular dynamics (QCMD) path-integral method, which was developed recently for computing the infrared spectra of condensed-phase systems. The main development is an improved estimator for the intermolecular torque on the quasicentroid. When applied to qTIP4P/F liquid water and ice, the new estimator is found to remove an artificial 25 cm$^{-1}$ red shift from the libration bands, to increase slightly the intensity of the OH stretch band in the liquid, and to reduce small errors noted previously in the QCMD radial distribution functions. We also modify the mass-scaling used in the adiabatic QCMD algorithm, which allows the molecular dynamics timestep to be quadrupled, thus reducing the expense of a QCMD calculation to twice that of Cartesian centroid molecular dynamics for qTIP4P/F liquid water at 300 K, and eight times for ice at 150 K.