Molecular Multipoles and (Hyper)Polarizabilities from Buckingham Expansion. Revisited

The Buckingham expansion is important for understanding molecular multipoles and (hyper)polarizabilities. In this study, we give a complete derivation of the Buckingham expansion in the traced form using successive Taylor series. Based on the derivation results, a general Buckingham expansion in the traced form is proposed, from which highly accurate numerical calculations using the finite field method can be achieved. The transformations from the traced multipoles and multipole-multipole polarizabilities to the corresponding traceless counterparts are realized with an auxiliary traced electric field gradient. The applications of the finite field method in this study show good agreements with previous theoretical calculations and experimental measurements.