PHASE TRANSITIONS OF TETRAHEDRAL MOLECULES USING MOLECULAR DYNAMICS SIMULATIONS

Abstract

High pressure studies are performed to probe the interatomic interactions in tetrahedral molecules, namely methane and carbon tetrafluoride. Within the framework of the isothermal.isobaric ensemble, and using a Lennard.Jones short.ranged interaction supplemented by the long.ranged Coulomb potential, molecular dynamics simulations (MD) are applied to study the dynamics of these systems. In regards to methane, its phase diagram is explored using MD simulations in the temperature range SOK to 300K, and pressures up to 30 GPa. We find that qualitatively and quantitatively our results are in good agreement --with-experimental findings in the literature. The structure of each phase is also investigated, and compared to the structures suggested by various experiments. Moreover, when applying the aforementioned procedure to carbon tetrafluoride molecules, a phase diagram is proposed which scans the temperature range 1 OOK.300K and up to 20 GPa. This phase diagram can resolve some of the contradictions between the experimental results and• explores some regions in which experiments have not yet been conducted. A new technique was adopted to describe the partial charges for modeling CF4 molecules; namely the partial equalization of orbital electronegativity. This method gives better agreement with the experimental melting point compared with the usual electric moment •methods.