Phase I occurs at low temperatures and pressures, and consists of a hexagonal close-packed array of freely rotating H2 molecules.
Furthermore, atomic structures can be inferred from a combination of experimental Raman spectra and first-principles modelling.
[7] Density functional theory calculations have been used to search for candidate atomic structures for each phase.
[8][9][10] Quantum Monte Carlo methods together with a first-principles treatment of anharmonic vibrational effects have then been used to obtain the relative Gibbs free energies of these structures and hence to obtain a theoretical pressure-temperature phase diagram that is in reasonable quantitative agreement with experiment.
[11] On this basis, Phase II is believed to be a molecular structure of P21/c symmetry; Phase III is (or is similar to) a structure of C2/c symmetry consisting of flat layers of molecules in a distorted hexagonal arrangement; and Phase IV is (or is similar to) a structure of Pc symmetry, consisting of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets.