Non-Orthogonal Determinant Multi-Slater-Jastrow Wave Functions in QMC

The efficiency of ab-initio quantum Monte Carlo (QMC) algorithms benefits greatly from compact variational trial wave functions that accurately reproduce ground state properties of a system. We investigate the possibility of using non-orthogonal determinants to create more compact wave functions than standard multi-Slater-Jastrow trial wave functions. As a test case, we compute variational and diffusion Monte Carlo (DMC) energies of a C₂ molecule. For a given multi-determinant expansion, we find that allowing the determinants to be non-orthogonal results in a fairly consistent ~ 0.4 eV improvement in the variational energy and ~ 0.2 eV improvement in the DMC energy. Our calculations indicate that trial wave functions with non-orthogonal determinants may noticeably improve computed energies in a QMC calculation when compared to their traditional orthogonal counterparts.