On the invariance of polarization propagators at SOPPA level of approach under unitary transformations of MOs

Abstract

Polarization propagators can be identified with linear responses of a system to external/internal perturbing fields, being that responses experimentally measurable. Then, approximated and reliable calculations with propagators requires to fulfill some well defined constraints like the invariance under unitary transformations, UT. Polarization propagators are not invariant under UT of molecular orbitals at second-order, known as SOPPA. We found the origin of such non invariance and propose a modification to partially cure this deficiency. Calculations with the original SOPPA model and the new one of NMR shieldings in small saturated and unsaturated systems show that differences appears for the unsaturated ones.