Higher RPA and second-order polarization propagator calculations of coupling constants in acetylene

Abstract

The higher random-phase approximation which includes electronic correlation at a level between the first-order (RPA, coupled Hartree—Fock) and the second-order polarization propagator approximation is found to remove the well-known deficiencies of RPA. In a numerical example (acetylene) the nuclear spin—spin coupling constants computed at the HRPA level even seem to be close to the second-order results. This is surprising since the excitation energies determined in HRPA often are too large. By examining the individual contributions, we conclude, that the numerical similarity found here between HRPA and the full second-order approximation may not always hold.