Variation of peak F-band absorption energy and lattice distortion with pressure has been calculated for lithium halides employing extended ion model and using values of local compressibility in the neighbourhood of the F-centres which include the effect of vacancy and pressure. The calculated results are in very good agreement with experimental results.

A suitable temperature dependence is introduced into the force parameters of the Lennard-Jones potential by taking the free-energy average of the Rowlinson potential. The resulting temperature-dependent parameters are used to calculate the effective potential, which is plotted for water vapour at a few different temperatures. The temperature-dependence of the second virial coefficient is shown for the cases of temperature-independent and temperature-dependent parameters of which the latter agrees very well with experimental results.

The modified free-energy averaged potential for water vapour is extended to NH₃ vapour. The resulting temperature-dependent parameters are used to compute the second virial coefficient of NH₃ vapour which agrees with experimental results particularly at higher temperatures.