Numerical analysis of the complex polarizability of the fixed-axis rotatoritinerant 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 comple x polarizability of the itinerant oscillator model indicates that an infini te number of oscillatory decay modes exist thus giving rise to an infinite number of harmonic peaks in the complex polarizability. The approximate for mula 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 bas ed on taking the first two terms of the Taylor series expansion of the corr elation function in terms of single transcendental functions. It is demonst rated by calculating numerically the Fourier transform of the correlation f unction 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 fo r the description of the model which also reduces the number of parameters needed to fit the model to experimental spectra is proposed. The polarizabi lity spectra calculated with the new parameter set from the model compare f avourably with the experimental spectra in methyl chloride.