EFFECT OF SHORT-RANGE REPULSIVE INTERACTIONS ON THE DYNAMICS OF THE METHANE MOLECULE

Using a computer simulation technique, an attempt has been made to explain the lambda-type transition in the specific heat of solid methane at around 20 K in terms of the changes in the dynamical behaviour of the methane molecule under the influence of its nearest neighbours. Different exponents of the short-range repulsive interaction occurring in the expression for the potential energy have been tried in order to select the appropriate value. The well known Lennard-Jones (6-12) and (6-15) potentials are found to reveal a phase transition in a well defined region. Rom an analysis of the direction cosine data, the three-dimensional motion of the central methane molecule has been visualised before and after the transition. Pertaining to the Lennard-Jones potential, the period of the torsional oscillation (libration) of the methane molecule comes out to be of the order of 0.3x10(-12) s. From the computed critical rotational kinetic energy, the transition temperature is found to be 20.2 K which agrees well with experimental observations.