Melting point determination from solid-liquid coexistence initiated by surface melting

A coexisting solid-liquid (s-l) system of nitromethane is created by surface-induced melting. A nitromethane crystal with a free surface is simulated by molecular dynamics (MD) in the constant-volume and -energy (NVE) ensemble for initial conditions generated by short MD simulations of the constant-volume and -temperature (NVT) ensemble at temperatures slightly above the melting point. Melting starts at the surface, initiating a solid-liquid interface, and the temperature drops as the system moves toward a state of equilibrium in which the solid and liquid phases coexist. The temperature at which the coexisting solid and liquid reach equilibrium is taken to be the melting point. The melting points of crystals with exposed (100), (010), and (001) crystallographic faces are predicted to be 238, 245, and 242 K, respectively. The predicted melting points are in good agreement with experiment (244.7 K) and previous simulations. The approach to equilibrium during the NVE simulation is monitored by calculating the orientational order parameter, diffusion coefficient, and density, which provide insights into the melting mechanism. The Sorescu-Rice-Thompson [J. Phys. Chem. B 2000, 104, 8406] force field, which accurately describes the inter- and intramolecular motions, was used. © 2007 American Chemical Society.