A correlated variational wave function for the orientational ground state of solid methane

We introduce a correlated variational wave function of the Jastrow type that is able to represent the ground state of solid CH4 through the orientational order–disorder transition. To describe the local orientation in a weakly ordered phase a new kind of pocket state is introduced, that is derived from the imaginary time propagator of a free top. The effect of short range correlations between the orientational zero point fluctuations is evaluated for pure meta (total spin=2) methane in the disordered and the ordered phases of the James and Keenan (JK) model. The T-A tunnel energy in fully converted methane is calculated in the same approximation and corrections to the mean field values are found to be large. While the difference in energy between the partially ordered phase II and the fully ordered JK phase III is found to be small, the latter remains marginally the more stable, in contrast to experiment.