Systematic incorporation of nuclear quantum effects into atomistic simulations by smoothed trajectory analysis

TL;DR

PIGSTA is a post-processing method that systematically incorporates nuclear quantum effects into atomistic simulations, improving convergence and accuracy without altering the original dynamics or incurring significant computational costs.

Contribution

The paper introduces PIGSTA, a novel post-processing framework that corrects finite-bead discretization errors in path-integral simulations, enabling accurate quantum effects inclusion with fewer beads.

Findings

PIGSTA recovers exact quantum limits for harmonic systems.

It significantly improves convergence of thermodynamic and structural properties.

PIGSTA provides a diagnostic for bead-number convergence.

Abstract

Nuclear quantum effects (NQEs) play an essential role in many atomistic systems, yet their explicit inclusion in molecular simulations remains challenging. Path-integral molecular dynamics (PIMD) provides a rigorous framework for incorporating NQEs, but its practical applicability is often limited by the slow and strongly system-dependent convergence with respect to the number of beads. Here we introduce path-integral generalized smoothed trajectory analysis (PIGSTA), a post-processing framework for the systematic incorporation of NQEs into atomistic simulations, using either classical or path-integral molecular dynamics trajectories. By applying analytically defined convolution kernels to simulation trajectories, PIGSTA corrects the frequency-dependent discretization error associated with a finite number of beads, without modifying the underlying dynamics. For harmonic systems, PIGSTA recovers the exact quantum-mechanical limit at any bead number, whereas standard PIMD becomes exact only in the infinite-bead limit. More generally, the method significantly improves the convergence of thermodynamic and structural observables at finite bead numbers and provides an internal, reference-free diagnostic of bead-number convergence based on the consistency of energy and force estimators. We assess PIGSTA for ambient liquid water and for the Zundel cation at ultralow temperature, representing a particularly demanding case for bead-number convergence. In both systems, PIGSTA reproduces the converged PIMD limit and enables physically consistent results at reduced bead numbers when convergence criteria are satisfied. Owing to its post-processing nature and negligible additional computational cost, PIGSTA offers a practical and broadly applicable approach for incorporating NQEs into atomistic simulations.