Stochastic effective core potentials, improving efficiency using a spin-dependent core definition

Numerically cheap single-core subsamplings have been used to build improved estimators for molecular properties in the variational Monte Carlo framework. The resulting estimators depend only on the valence electron positions and can be thought of as an exact effective core potential for the total energy. We are proposing a spin-dependent core definition which enables exploiting these single-core subsamplings (or sidewalks) not only to decrease the variance of the estimators but also to restrict the main variational Monte Carlo dynamics to the valence region. This results mainly in a simplification of the algorithm and additionally in a gain in efficiency as illustrated on alkane chains and silicon clusters. An evaluation of the efficiency on transition metal systems is done using cobalt clusters, a gain of up to two orders of magnitude is achieved compared to a standard all-electron calculation.