Vibrational and thermal effects on the dipole polarizability of methane and carbon tetrachloride from vibrational structure calculations.

We present a theoretical study of vibrational and thermal effects on the dipole polarizability of methane and carbon tetrachloride. Using a fourth order Taylor expansion in rectilinear normal coordinates of the potential and property surfaces we solve the vibrational problem using vibrational structure theory, e.g., through vibrational self-consistent-field or vibrational configuration-interaction theory. For each vibrational state we calculate in addition the vibrational state average polarizability. Constructing the vibrational partition function by "brute force" allows for prediction of thermal effects on the dipole polarizability. The method is not restricted in any way to polarizabilities nor to the specific representation of the potential and property surfaces employed in this work. Any molecular property with a suitable normal coordinate representation may be considered. We discuss the performance of vibrational self-consistent field as compared to vibrational configuration interaction and study in detail the convergence of the former method with respect to the number of vibrational states included in the thermal averaging. Based on calculations including up to 170,000 vibrational self-consistent-field states we present thermal effects on the dipole polarizability of methane and carbon tetrachloride in the temperature ranges 0-1100 and 0-500 K, respectively. The predicted thermal effect on the dipole polarizability of methane is found to be approximately 0.8% which compare well with previous experimental measurements.