Correlation between electronic and molecular structure distortions and vibrational properties. I. Adiabatic approximations

Distortions of the electronic and molecular structures of a polarizable molecule in an inhomogeneous electrostatic potential were investigated by using perturbation theory. The second-order electronic energy correction term is directly related to the charge response kernel, which is a site representation of the nonlocal charge response susceptibility. Instead of using the sum-over-state expression for the charge response kernel, we discuss how a finite-field method using point charges can be used to calculate the charge response kernel. By invoking various levels of adiabatic approximations, four different coupled differential equations for the vibrational wave functions were obtained. From the effective vibrational potential functions thus obtained, the molecular structural distortion induced by the external electrostatic potential was shown to be calculable. We also present a discussion on how vibrational properties are affected by the presence of the electrostatic potential. The relationship between the vibrational frequency shift and molecular structural distortion, when a polarizable molecule is exposed to an electrostatic potential, was elucidated. (C) 2003 American Institute of Physics.