CCQ Quantum Cafe with David Ceperly

Title: The role of QMC in developing Born-Oppenheimer potentials: results on dense hydrogen

Abstract:

Because continuum Quantum Monte Carlo methods such as Diffusion MC and Path Integral MC are directly formulated in coordinate space, they do not have the limitation of a basis set which helps in disordered systems such as those liquids, chemical reactions, solids with large amplitude motions, and surfaces. Slater-Jastrow-backflow wavefunctions are a compact, efficient and understandable description of systems with strong correlation. An important application for QMC is to provide unbiased data for bespoke force-fields and has advantages over other accurate electronic structure methods such as those based on DFT.

As an example I will discuss recent calculations of hydrogen under extreme conditions of temperature and pressure. This system is important both because of its ubiquitous presence in the universe and in technological applications. Even though it is the “holy grail” of high pressure studies, its phase diagram above 100 GPa is uncertain. We have made a new study of its phase diagram using a machine-learned interatomic potential trained with Quantum Monte Carlo forces and energies and we find two new stable phases. The high temperature phase has a reentrant melting line with a maximum at higher temperatures (1450K at 150GPa) than previously estimated.