The molecular phase diagram of carbon dioxide by molecular simulations of the TraPPE model

Abstract

The determination of the phase diagram of carbon dioxide by using exclusively experimental techniques has been prevented by the strong metastabilities of solid phases and the hysteresis in some phase transitions. Quantum mechanical methods have played a crucial role in this field helping the characterization of the molecular/non molecular character of the phases. Nowadays, it is well-established that CO2-I, CO2-II, CO2-III (first termed as VII), and CO2-IV phases are molecular phases in contrast with high temperature and high pressure non-molecular phases such as CO2-V and CO2-VI. In this work, we explore the ability of the molecular force field TraPPE to describe the molecular part of the carbon dioxide phase diagram by using classical Monte Carlo and molecular dynamics simulations. As it will be shown, this model predicts very accurately the fluid-I transition line and the stability of phases CO2-I, CO2-II, CO2-III, and CO2-IV in well-defined regions of the phase diagram. The main shortcomings are the small size of the stability regions of CO2-II and CO2-III and the appearance of CO2-II, CO2-III, and CO2-IV phases at too low pressures. In addition, the densities of the solid phases are only well-predicted for CO2-I and at pressures lower than 3 GPa. At higher pressures, the density is underestimated for all the solids. The results clearly indicate that although the TraPPE model does a reasonable job in describing the phase diagram of CO2, there is still room for improvement.