ORIENTATIONAL ORDER IN DENSE MOLECULAR-HYDROGEN - A 1ST-PRINCIPLES PATH-INTEGRAL MONTE-CARLO CALCULATION

A quantum-mechanical simulation of orientational order in solid molecular hydrogen is presented, based on path-integral Monte Carlo calculations. The effective potential between nearest-neighbor hydrogen molecules is derived from first-principles total energy calculations, taking full account of the symmetries of the Hamiltonian within a hexagonal close-packed arrangement of the molecule centers. A first-order orientational order transition is found by studying short- and long-range angular and the behavior of the corresponding order parameters. A pressure-temperature phase diagram is constructed and compared to relevant experimental and theoretical results.