Exact Calculation of Quantum Thermal Average from Continuous Loop Path Integral Molecular Dynamics

The quantum thermal average plays a central role in describing the thermodynamic properties of a quantum system. From the computational perspective, the quantum thermal average can be computed by the path integral molecular dynamics (PIMD), but the knowledge on the quantitative convergence of such approximations is lacking. We propose an alternative computational framework named the continuous loop path integral molecular dynamics (CL-PIMD), which replaces the ring polymer beads by a continuous loop in the spirit of the Feynman--Kac formula. By truncating the number of normal modes to a finite integer $N\in\mathbb N$, we quantify the discrepancy of the statistical average of the truncated CL-PIMD from the true quantum thermal average, and prove that the truncated CL-PIMD has uniform-in-$N$ geometric ergodicity. These results show that the CL-PIMD provides an accurate approximation to the quantum thermal average, and serves as a mathematical justification of the PIMD methodology.