Effects of the order of the energy asymptotes on the calculations of nuclear magnetic shieldings and static polarizabilities

Abstract

The second-order polarization propagator approximation (SOPPA) has been applied to the calculation of the nuclear magnetic shielding constants and static polarizabilities of CO, N2, F2, and CH4 in order to investigate the effect of decreasing the order (in perturbation theory) of the poles of the propagator, i.e., the energy asymptotes, but still requiring that all response terms to second order must be included. Our results show that the higher than second-order contributions from the poles are of vital importance for the nuclear magnetic shieldings and of less, but not negligible, importance for the static polarizabilities. As the order of the poles is decreased the SOPPA isotropic shieldings approach the results obtained within second-order perturbation theory (MP2), especially for CO, N2, and F2. This behavior is not as pronounced for the C shielding of CH4 and for polarizabilities. For the shieldings we obtain the best agreement with MP2 (and experiments) when the poles are calculated as Hartree–Fock energy differences using frozen ground state orbitals, and for the polarizabilities when the poles are calculated in the random phase approximation and Tamm–Dancoff approximation.