Thermodynamics using effective spherical potentials for CO2 anisotropies

Abstract

We examine the fluid thermodynamics of a model homonuclear diatomic system with anisotropies characteristic of CO2. The density (CO2 densities) and temperature regime is 1.6 g/cm3≲ρ≲2.6 g/cm3 and 1000 K≲T≲7000 K. Extensive molecular dynamics data for the model equation of state are presented. Comparisons are then made to the thermodynamics from three effective spherical potentials; the potential median, the radial median, and an exponential-six with parameters adjusted to best fit the true thermodynamics. The two median potentials typically give 3% agreement for the higher temperature fluid with a 5%–10% comparison nearer the freezing line for both pressure and internal energy while the fit is good to 3% or better. Thus there exists an effective spherical potential that very accurately models the thermodynamics of dense fluid CO2, a system whose potential energy in the repulsive region varies by three to four orders of magnitude as a function of angles with fixed center of mass separation. The median averages give an excellent representation of this effective spherical potential.