How Important is Orbital Choice in Single-Determinant Diffusion Quantum Monte Carlo Calculations?

The accuracy of total electronic energies obtained using the fixed-node diffusion quantum Monte Carlo (FN-DMC) method is determined by the choice of the many-body nodal surface. Here, we perform a systematic comparison of the quality of FN-DMC energies for a selection of atoms and diatomic molecules using nodal surfaces defined by single determinants of Hartree-Fock), B3LYP, and LDA orbitals. Through comparison with experimental results, we show that the use of Kohn-Sham orbitals results in significantly improved FN-DMC atomization energies over those obtained using Hartree-Fock orbitals. We also discuss the effect of spin contamination in the orbitals.