Oral: Improving the Efficiency of Path Integral Monte Carlo for Quantum Solids and Confined Liquids

We propose an improved Path Integral Monte Carlo (PIMC) algorithm called Harmonic PIMC (H-PIMC). PIMC is a powerful tool for studying quantum condensed phases. However, it often suffers from a low acceptance ratio for solids and dense confined liquids. We develop a sampling scheme especially suited for such problems by dividing the potential into its harmonic and anharmonic contributions. We generate the imaginary time paths for the harmonic part of the potential exactly and accept or reject it based on the anharmonic part. We benchmark the method on systems with increasing anharmonicity, improving the acceptance ratio by a factor of 2-4, and significantly lowering the auto-correlation time. We also find that the method requires a smaller number of imaginary time slices for convergence, which leads to another two- to four-fold acceleration at the lowest temperatures. Finally, we successfully apply the method to systems with multiple metastable states by updating the harmonic part based on the particle location. H-PIMC will improve the efficiency of simulating quantum solids and dense confined liquids at the cost of a single (or a few) hessian evaluations.