Correlation times and complex permittivity for linear and spherical top molecules

We describe a simple method for calculating correlation times and complex permittivities from the orientational autocorrelation functions of molecules executing rotational Brownian motion. The expression for the autocorrelation function is first rearranged to display explicitly its asymptotic form for large times. We apply the method to recent results on the autocorrelation functions for linear and spherical-top molecules.