DYNAMICS AND PHASE-TRANSITIONS IN SOLID ORTHO-HYDROGEN AND PARA-HYDROGEN AND DEUTERIUM FROM AN ABINITIO POTENTIAL

Starting from an ab initio H2-H2 potential that contains anisotropic short-range and dispersion terms, in addition to the quadrupole-quadrupole interactions, we have performed lattice-dynamics calculations for the orientationally disordered hexagonal (hcp) and ordered cubic (Pa3) phases of solid (ortho and para) hydrogen and deuterium. The method used is the time-dependent Hartree (TDH) formalism, with the explicit inclusion of translation-rotation coupling. By an anharmonic expansion of the potential through sixth order in the molecular displacements and the use of wave functions for the translational vibrations that are sufficiently flexible to adapt to this strong anharmonicity, we could avoid the usual (effective) Jastrow correction to the potential. The calculated phonon and roton or libron frequencies are in fairly good agreement with infrared, Raman, and neutron-scattering data, significantly better in general than the results from earlier (separate) phonon calculations and roton or libron calculations that have used empirical potentials. The transition pressure for ordering para-H2 or ortho-D2 appears to be dominated by the classical quadrupole-quadrupole interactions. It is significantly affected by the increase of the rotational constant and, especially, by the reduction of the quadrupole moment, which follows from a shortening of the intramolecular bond. Translation-rotation coupling yields the observed mixing of phonons and rotons at high pressure, but its effect on the transition pressure is minute. The remaining discrepancy between the calculated and observed transition pressures must be caused by three-body interactions and by correlations between the molecular motions that are beyond the TDH approximation. © 1990 The American Physical Society.