Theory of vibronic interactions: The importance of floating basis sets

Abstract

The theory of vibronic interactions in polyatomic molecules is investigated in the nondegenerate case and the importance of using a floating basis set when expanding the electronic eigenfunction as a function of nuclear position is demonstrated. Normal Herzberg–Teller theory using a fixed basis set considers only interactions with the vibrating nuclei. The physical meaning of the extra terms in Liehr’s formalism, which uses a floating basis set, is found to be interactions with electron distributions moving with the nuclei. Furthermore, it is shown that Liehr’s formalism can be simplified if electron coordinates are related directly to nuclear position. Although a floating basis set is used to develop our equations, only unperturbed zero order electronic functions for the equilibrium nuclear configuration need be known to carry out most computations involving vibronic wavefunctions in our new formalism. This simplifies calculations considerably.