Isothermal-isobaric ensemble simulations of melting in quantum solids

An isothermal-isobaric ensemble version of the Fourier path-integral Monte Carlo method is formulated and applied to study melting in quantum solids. Experimental molar volumes of solid ortho-D-2, para-H-2, and HD an shown to be reasonably well reproduced by simulations using a Lennard-Jones potenti al. The fee and hcp solid structures of ortho-D-2 are compared at 14 K and 0.85 MPa. The large quantum effects in solid D-2 are demonstrated by the fa ct that the quantum solid at 14 K and 0.85 MPa has an average kinetic energ y per particle which is 3.2 times its classical value and a density which i s 21% less than that for the corresponding classical solid. The 0.85 MPa is obar, starting with an fee solid at 14 K, is mapped out between 14 and 30 K . Changes in the various structural and energetic quantities with temperatu re have been monitored as a function of temperature. The instantaneous norm al-mode spectrum of the quantum system is followed as a function of tempera ture and shown to undergo significant changes on melting of the quantum sol id. In particular, the Einstein frequency and the average participation rat io of the real branch is shown to decrease sharply on melting while the fra ction of imaginary frequencies increases.