Vibrational averaging of electrical properties

Abstract

A routine method is described for incorporating the effects of vibrational averaging into theoretical predictions of electric properties of polyatomic molecules. The method relies upon perturbation theory expressions for the vibrational correction in terms of derivatives of the energy and property surfaces, and utilizes a least-squares approach to computing the necessary force constants and property derivatives. Least-squares derived derivatives are compared with those obtained using analytical methods, and zero-point vibrational corrections are presented for the first-row hydrides CH₄, NH₃, H₂O, and HF, for the dipole and quadrupole moments and the components of the static dipole polarizability, at both SCF and MP2 levels of theory. The zero-point vibrational corrections obtained are shown to be in good agreement with the limited number of previous theoretical results for these molecules, and the electric moments corrected for vibrational effects are in excellent agreement with experiment. Vibrational averaging is shown to be especially important for the polarizability anisotropy, and excellent agreement with experiment is obtained. However, the predicted mean polarizabilities are uniformly greater than experiment, only partly due to the limited correlation treatment used. The overall success of the method applied to these simple polyatomic molecules demonstrates the feasibility of routinely incorporating vibrational effects in studies which aim to critically compare theory and experiment.