Generalized Path Integral Energy and Heat Capacity Estimators of Quantum Oscillators and Crystals using Harmonic Mapping

TL;DR

This paper introduces generalized path integral estimators using harmonic mapping to improve the precision of energy and heat capacity calculations in quantum oscillators and crystals, enhancing computational efficiency.

Contribution

It develops harmonic mapping-based estimators that outperform standard methods in accuracy for quantum systems, applicable to both anharmonic oscillators and crystalline solids.

Findings

HMAq estimators outperform CVir in precision.

HMAq-NM provides the best accuracy among tested methods.

Estimators are effective for systems with anharmonicity and quantum effects.

Abstract

Imaginary-time path integral (PI) is a rigorous tool to treat nuclear quantum effects in static properties. However, with its high computational demand, it is crucial to devise precise estimators. We introduce generalized PI estimators for the energy and heat capacity that utilize coordinate mapping. While it can reduce to the standard thermodynamic and centroid virial (CVir) estimators, the formulation can also take advantage of harmonic character of quantum oscillators and crystals to construct a coordinate mapping. This yields harmonically mapped averaging (HMA) estimators, with mappings that decouple (HMAc) or couple (HMAq) the centroid and internal modes. The HMAq is constructed with normal mode coordinates (HMAq-NM) with quadratic scaling of cost or harmonic oscillator staging (HMAq-SG) coordinates with linear scaling. The estimator performance is examined for a 1D anharmonic oscillator and a 3D Lennard-Jones crystal using path integral molecular dynamics (PIMD) simulation. The HMA estimators consistently provide more precise estimates compared to CVir, with the best performance obtained by HMAq-NM, followed by HMAq-SG, and then HMAc. We also examine the effect of anharmonicity (for AO), intrinsic quantumness, and Trotter number. The HMA formulation introduced assumes the availability of forces and Hessian matrix; however, an equally efficient finite difference alternative is possible when these derivatives are inaccessible. The remarkable improvement in precision offered by HMAq estimators provides a framework for efficient PI simulation of more challenging systems, such as those based on \textit{ab initio} calculations.