Hybridizing Pseudo-Hamiltonians and Non-local Pseudopotentials in Diffusion Monte Carlo

Projector quantum Monte Carlo (QMC) methods are among the most accurate many body techniques to query the properties of the electronic ground state. Due to computational efficiency, non-local pseudopotentials (NLPPs) are used in QMC is for all but light elements. This comes at the price of localization approximations (LAs) that can degrade total accuracy. An alternate pseudo-Hamiltonian (PH) approach that does not require LA was considered early in the QMC history of core pseudization [PRL 62 2088 (1989)], but was not adopted as producing adequate potentials was difficult. In this work, we explore the hybridization of NLPPs and PHs in an attempt to reduce the non-local components. For 3d transition metals we show that hybrid PHs can be as accurate as NLPPs, but with a much smaller non-local part. We find that a simple approach to partitioning scattering channels between the NLPP and PH compoents does not lead to a reduction of localization error, but instead aligns the behavior of the prevailing locality and T-moves approximations. Reasons for this behavior and possible avenues for direct minimization of localization error are discussed.