Orientational phase transitions in molecular N2 solids:: A path-integral Monte Carlo study

A molecular crystal composed of rigid N-2 molecules interacting via Lennard-Jones and electrostatic interactions is studied by path-integral Monte Carlo simulations in the constant pressure ensemble. The simulation scheme employed takes fully into account quantum effects on both translational and rotational degrees of freedom of the molecules. The effect of quantum fluctuations on molar volumes, energies, and transition temperatures is studied for different values of the external pressure. At zero pressure the transition temperature from a high-temperature orientationally disordered cubic phase to a low-temperature phase with Pa3 structure is reduced by about 11% due to quantum delocalization. With increasing Trotter number, the molar volume and energy at low temperatures approach the experimentally observed values, in contrast to the classical simulations. As the pressure increases, the transition temperature is shifted to larger values and the difference between the classical and quantum values is decreasing. [S0163-1829(98)07238-5].