The S0(0) structure in highly compressed hydrogen and the orientational transition

A calculation of the rotational S0(0) frequencies in high-pressure solid parahydrogen is performed. Convergence of the perturbative series at high density is demonstrated by the calculation of second- and third-order terms. The results of the theory are compared with the available experimental data to derive the density behaviour of structural parameters. In particular, a strong increase of the value of the lattice constant ratio c/a and of the internuclear distance is determined. Also a decrease of the anisotropic intermolecular potential is observed which is attributed to charge transfer effects. The structural parameters determined at the phase transition may be used to calculate quantum properties of the rotationally ordered phase.