The Schr\"odinger formulation of the Feynman path centroid density

TL;DR

This paper analyzes the Feynman path centroid density to deepen understanding of its connection with the Schrödinger formulation, revealing its role in approximating quantum excitation energies and effective potentials.

Contribution

It provides a new analysis linking the path centroid density with the Schrödinger formulation, enhancing the theoretical foundation of path integral methods in quantum statistical mechanics.

Findings

Centroid density relates to the quasi-static response of quantum systems.

Path centroid dispersion approximates excitation energies at low temperatures.

Zero temperature limit yields probability densities of minimum energy wave packets.

Abstract

We present an analysis of the Feynman path centroid density that provides new insight into the correspondence between the path integral and the Schr\"odinger formulations of statistical mechanics. The path centroid density is a central concept for several approximations (centroid molecular dynamics, quantum transition state theory, and pure quantum self-consistent harmonic approximation) that are used in path integral studies of thermodynamic and dynamical properties of quantum particles. The centroid density is related to the quasi-static response of the equilibrium system to an external force. The path centroid dispersion is the canonical correlation of the position operator, that measures the linear change in the mean position of a quantum particle upon the application of a constant external force. At low temperatures, this quantity provides an approximation to the excitation energy of the quantum system. In the zero temperature limit, the particle's probability density obtained by fixed centroid path integrals corresponds to the probability density of minimum energy wave packets, whose average energy define the Feynman effective classical potential.