ORIENTATIONAL PHASE-TRANSITIONS IN MOLECULAR N-2 SOLIDS - A PATH-INTEGRAL MONTE-CARLO STUDY

A molecular crystal composed of rigid N-2 molecules interacting via Le nnard-Jones and electrostatic interactions is studied by path-integral Monte Carlo simulations in the constant pressure ensemble. The simula tion 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 transit ion temperatures is studied for different values of the external press ure. At zero pressure the transition temperature from a high-temperatu re orientationally disordered cubic phase to a low-temperature phase w ith Pa3 structure is reduced by about 11% due to quantum delocalizatio n. 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 transitio n temperature is shifted to larger values and the difference between t he classical and quantum values is decreasing. [S0163-1829(98)07238-5] .