SOLID MOLECULAR-HYDROGEN - THE BROKEN SYMMETRY PHASE

By performing constant-pressure variable-cell ab initio molecular dyna mics simulations we find a quadrupolar orthorhombic structure of Pca2( 1) symmetry for the broken symmetry phase (phase II) of solid H-2 at T = 0 and P = 110-150 GPa. We present results for the equation of state , lattice parameters, and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H-2 and D-2. We assign the observed modes to specific symmetry representation s.