DETERMINATION OF AN EFFECTIVE INTERMOLECULAR POTENTIAL FOR CARBON-DIOXIDE USING VAPOR LIQUID-PHASE EQUILIBRIA FROM NPT PLUS TEST PARTICLE SIMULATIONS

Molecular simulations according to the NpT + test particle method are used for the determination of vapour-liquid phase equilibria of six mo del potentials of the two-centre Lennard-Jones plus point quadrupole ( 2CLJQ) type. By fitting to experimental saturation properties of carbo n dioxide, an optimized effective 2CLJQ potential for this substance i s found to have the parameters is-an-element-of/k 125.317 K, sigma = 3 .0354 angstrom, L = l/sigma = 0.699, and Q2 = Q/is-an-element-of sigm a5 = 3.0255; the resulting quadrupole moment Q differs from the most r ecent experimental value by only 9%. Using this potential, densities a nd enthalpies of carbon dioxide are obtained again by molecular simula tions as functions of temperature and pressure in large parts of the f luid region extending from 230 to 570 K, and pressures up to 400 MPa. The results show that effective potentials obtained by fitting to expe rimental saturation properties also yield good predictions of the ther modynamic properties for polar substances.