Solid Molecular Hydrogen: The Broken Symmetry Phase

By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure of ${\mathrm{Pca}2\mathrm{}}_1$ symmetry for the broken symmetry phase (phase II) of solid ${\mathrm{H}}_2$ at $T=0\mathrm{}$ and $P=110\mathrm{}–150\mathrm{GPa}$. 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 ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. We assign the observed modes to specific symmetry representations.