Melting points and thermal expansivities of proton-disordered hexagonal ice with several model potentials

Authors

Yuji Koyama, Okayama University
Hideki Tanaka, Okayama UniversityFollow
Guangtu Gao, University of Nebraska-Lincoln
Xiao Cheng Zeng, University of Nebraska-LincolnFollow

Date of this Version

10-22-2004

Comments

Published by American Institute of Physics. J. Chem. Physics VOLUME 121, NUMBER 16, 22 OCTOBER 2004. ©2004 American Institute of Physics. Permission to use. http://jcp.aip.org/.

Abstract

A method of free energy calculation is proposed, which enables to cover a wide range of pressure and temperature. The free energies of proton-disordered hexagonal ice (ice Ih) and liquid water are calculated for the TIP4P [J. Chem. Phys. 79, 926 (1983)] model and the TIP5P @J. Chem. Phys. 112, 8910 (2000) model. From the calculated free energy curves, we determine the melting point of the proton-disordered hexagonal ice at 0.1 MPa (atmospheric pressure), 50 MPa, 100 MPa, and 200 MPa. The melting temperatures at atmospheric pressure for the TIP4P ice and the TIP5P ice are found to be about T_m=229 K and T_m=268 K, respectively. The melting temperatures decrease as the pressure is increased, a feature consistent with the pressure dependence of the melting point for realistic proton-disordered hexagonal ice. We also calculate the thermal expansivity of the model ices. Negative thermal expansivity is observed at the low temperature region for the TIP4P ice, but not for the TIP5P ice at the ambient pressure.