Ab initio molecular orbital calculations by the resonating Hartree-Fock approach: superposition of non-orthogonal Slater determinants

The resonating Hartree-Fock (ResHF) method is applied to ab initio MO calculations of CO. This method approximates correlated wavefunctions by superposition of non-orthogonal Slater determinants, where both the CI coefficients and the orbitals in the determinants are variationally determined. The ResHF, with two or three generating determinants, explains the correlation energy to the same extent as the full-valence CASSCF wavefunction with 328 CSFs. Furthermore, the method reproduces the dipole moment more accurately, reflecting a better long-range behavior of the wavefunction. This implies the ResHF method provides more suitable reference functions in multireference methods.