On the equation of state and pressure dependence of optical phonon frequencies for solidified o-D2 and p-H2 in the self-consistent phonon approximation

Phonon dispersion curves for hcp o-D2 and p-H2 have been reported by Nielsen and Bjerrum-Moeller and Nielsen. In addition the Raman active optical phonons have been obtained by Silvera et al. and Berkhout and Silvera. So far, theoretical calculations for these quantities have been reported by Klein and Koehler and Mertens and Biem. The calculated dispersion relations deviated by as much as 25 to 35% from the measured values. These calculations made use of the Jastrow factor formalism in order to better take into account short range correlations. However they did not contain important higher order anharmonic corrections to the self-energy, such as the 3-phonon term. In order to give an estimate of the magnitude of these anharmonic corrections, and specifically their pressure dependence, selected preliminary results on fcc o-D2 and p-H2 are reported. These results should be relevant to hcp because, for the case of pairwise-additive intermolecular interactions (a) calculated bulk quantities such as the free energy differ by less than 0.1% between fcc and hcp, and (b) fcc phonon dispersion curves in the (111) direction calculated in the quasiharmonic approximation (QH) or self-consistent phonon approximation SC1 (without 3-phonon corrections) map one-to-one onto those of hcp along the c-axis. These QH or SC1 calculations show these hcp/fcc frequencies to agree to better than 0.1% in o-D2 using a Lennard-Jones potential at p=O. (Auth.)