The Mori formalism,a critical discussion of its use in representing far infrared and microwave absorptions

The three variable Mori formalism for molecular dynamics in the fluid state is evaluated using some extensive data of Gerschel's on three strongly dipolar symmetric tops in the microwave and far infrared region of the electromagnetic spectrum. It is found that the use of loss and power absorption coefficients can lead to misleading results when theory and experiment are compared and it is essential in general to get the data over the whole frequency range of interest. Using this procedure it is found that the second memory function of the Mori continued fraction is difficult to approximate satisfactorily with a simple decay (such as an exponential) and it is unlikely that successive memory functions of the total orientational correlation function are simpler functions of time than the latter.