Structure and pair correlations of a simple coarse grained model for supercritical carbon dioxide

A recently introduced coarse-grained pair potential for carbon dioxide molecules is used to compute structural properties in the supercritical region near the critical point, applying Monte Carlo simulations. In this model, molecules are described as point particles, interacting with Lennard-Jones (LJ) forces and a (isotropically averaged) quadrupole-quadrupole potential, the LJ parameters being chosen such that gratifying agreement with the experimental phase diagram near the critical point is obtained. It is shown that the model gives also a reasonable account of the pair correlation function, although in the nearest neighbour shell some systematic discrepancies between the model predictions and results from simulations of fully atomistic models and experiments are found. By comparison with results from an equivalent model but with the full angle-dependent quadrupolar interaction these discrepancies are traced back to the effect of orientational correlations and of the insufficient representation of molecular packing. Furthermore, the correlation length of density fluctuations is obtained from Ornstein-Zernike plots of the inverse structure factor, and shown to be in rough agreement with corresponding experimental results. Finally possible refinements of this coarse-grained model are briefly discussed.