ROTATIONAL ORDERING IN SOLID DEUTERIUM AND HYDROGEN - A PATH-INTEGRALMONTE-CARLO STUDY

The path-integral Monte Carlo method with a constant-pressure ensemble is used to study both translational and orientational transitions in the phase diagram of D-2 and H-2 up to megabar pressures. With an inte rmolecular interaction potential determined to agree with the experime ntal equation of state, a rotational order-disorder phase transition i s observed. The phase line for this transition is in quantitative agre ement with part of the phase diagram for both D-2 and H-2. No structur al phase transition, and no transitions to the D-A and H-A phases (pha se III) are observed. We attribute this in part to the limitations of simulation cell size.