Property evaluation and orbital relaxation in coupled cluster methods

Abstract

Molecular electronic properties such as dipole moments, polarizabilities and hyperpolarizabilities and quadrupole moments and polarizabilities, and spin properties such as hyperfine splitting constants and nuclear magnetic coupling constants are predicted by ab initio coupled cluster (CC) methods for a variety of molecules. We compare the results of property evaluation using orbitals that have been allowed to relax in the presence of the perturbation and results obtained using nonrelaxed orbitals. It is demonstrated numerically, and proven formally, that the coupled cluster singles and doubles (CCSD) model using nonrelaxed orbitals is able to include most of the relaxation effects for the evaluation of first- and second-order properties. Thus there is little reason to perform coupled (perturbed) Hartree–Fock calculations as a precursor to correlated CCSD calculations of such properties.