Study of the structures of solid hydrogen at megabar pressures by means of first-principles calculations

Abstract

A calculation of the Raman- and infrared-active vibrational modes in compressed solid hydrogen is presented and the structures of solid hydrogen at megabar pressures are discussed in the light of the vibrational frequencies and the total energies. We performed band theoretical calculations in the local density approximation (LDA) using a plane-wave basis. The frozen-phonon method was used in the calculation of the vibrational frequencies, where we characterized the modes using group theoretical analysis. The effects of band-gap closure on the vibrational frequencies and those of the gradient correction to the LDA (the generalized-gradient approximation, GGA) for the exchange-correlation energy are also studied. In the structure of the hexagonal close-packed (hcp) lattice for the molecular centres, the mid-lying phonon modes whose frequencies lie in the range at 100-200 GPa and the frequency difference between the in-phase and out-of-phase vibronic modes are in good agreement with experiments. The present study suggests that the most probable lattice of the solid hydrogen is hcp below and beyond the phase transition at 150 GPa, and that the molecules are canted away from the c-axis.