Thermodynamic properties for a simple model of solid carbon dioxide: Monte Carlo, cell model, and quasiharmonic calculations

Abstract

Monte Carlo, cell model, and quasiharmonic calculations are employed to investigate a molecular crystal model consisting of rigid linear molecules disposed in the cubic P a 3 structure and interacting with pair potentials between the ends. The errors involved in employing the cell model and quasiharmonic approximations are established. The thermodynamic properties for a model of solid CO₂ are calculated as functions of temperature for a variety of 6—n potentials, including those obtained from previous lattice dynamical calculations in which fits were made to observed phonon frequencies. The results are compared with experimental measurements of the sublimation energy, heat capacity, bulk modulus, Grüneisen function, and thermal expansion. The best over‐all fit was obtained with a 6–9 potential. However, a consistent inadequacy of this interaction model is that it exhibits too much thermal expansion for any reasonable fit to the other properties.