Non-orthogonal multi-Slater determinant expansions in auxiliary field quantum Monte Carlo

We investigate the use of non-orthogonal multi-Slater determinant (NOMSD) expansions as trial wavefunctions in auxiliary field quantum Monte Carlo simulations of molecular systems. We show that NOMSD trial wavefunctions with as few as twenty determinants are sufficient in order to achieve chemical accuracy across most of the G1 molecular test set. We also show that NOMSD trial wavefunctions are useful for more challenging strongly correlated systems by computing relative energies along the isomerization path of the [Cu₂O₂]²⁺ molecule. Our results for [Cu₂O₂]²⁺ compare favorably with other accurate quantum many-body methods, including density matrix renormalization group and completely renormalized coupled cluster methods.