Semi-stochastic full configuration interaction quantum Monte Carlo: Developments and application.

We expand upon the recent semi-stochastic adaptation to full configuration interaction quantum Monte Carlo (FCIQMC). We present an alternate method for generating the deterministic space without a priori knowledge of the wave function and present stochastic efficiencies for a variety of both molecular and lattice systems. The algorithmic details of an efficient semi-stochastic implementation are presented, with particular consideration given to the effect that the adaptation has on parallel performance in FCIQMC. We further demonstrate the benefit for calculation of reduced density matrices in FCIQMC through replica sampling, where the semi-stochastic adaptation seems to have even larger efficiency gains. We then combine these ideas to produce explicitly correlated corrected FCIQMC energies for the beryllium dimer, for which stochastic errors on the order of wavenumber accuracy are achievable.N.S.B. gratefully acknowledges Trinity College, Cambridge for funding. J.S.S. acknowledges the research environment provided by the Thomas Young Centre under Grant No. TYC-101. G.H.B. gratefully acknowledges the Royal Society for a university research fellowship. This work has been supported by the EPSRC under grant no. EP/J003867/1.This is the author accepted manuscript. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/jcp/142/18/10.1063/1.4920975