Numerical analysis of the complex polarizability of the fixed-axis rotator itinerant oscillator model:: comparison with experimental results of the Debye and far infrared absorption of methyl chloride

The double-transcendental nature of the correlation function for the complex polarizability of the itinerant oscillator model indicates that an infinite number of oscillatory decay modes exist thus giving rise to an infinite number of harmonic peaks in the complex polarizability. The approximate formula for the complex polarizability given by Coffey and Walsh (1997 J. Chem. Phys. 106 7625) does not include these decay modes as this formula is based on taking the first two terms of the Taylor series expansion of the correlation function in terms of single transcendental functions. It is demonstrated by calculating numerically the Fourier transform of the correlation function that the approximate formula provides an accurate approximation to the complex polarizability since the contribution of the higher-order decay modes is negligible. In addition, a new dimensionless set of parameters for the description of the model which also reduces the number of parameters needed to fit the model to experimental spectra is proposed. The polarizability spectra calculated with the new parameter set from the model compare favourably with the experimental spectra in methyl chloride.