fcc-hcp Transition in Solid Orthohydrogen

The cluster-variation method is applied to calculate the orientational ordering of $o-{\mathrm{H}}_2$ molecules on fcc and hcp lattices. The $\mathrm{Pa}3\mathrm{}$, $\mathrm{Pc}{a2\mathrm{}}_1$, and $\frac{{P2\mathrm{}}_1}{c}$ structures are examined. A fcc-hcp structural transition very near to an orientational ordering transition is predicted. These transitions result because of the anisotropic intermolecular interactions. The isotropic interactions are expected to play only a very minor role.