Radial distribution functions of molecular systems at low density Original Research Article

The site-site radial distribution functions g(r) of chlorine and heavy water, modelled by potentials which are in use in the computer simulations, are calculated in the zero density limit. The comparison with the available experimental data in the gas phase is performed. In the case of chlorine the reliability of effective potentials is tested and the role played by preferred molecular orientations is studied. In the case of heavy water, the calculations exploiting effective pair potentials yield g(r) which severely overestimate the experimental one at 580 K and 0.031 g/cm3. The agreement improves if the ab initio MCY potential is used. By contrast, a surprisingly good agreement is found between the experimental data at 0.052 g/cm3 and the predictions of the TIP4P potential model. By comparing chlorine and heavy water results, the low density behaviour of experimental g(r) is explained in terms of many-body effects. In particular, evidence of non-additive intermolecular forces is obtained. Furthermore, from the analysis of the partial atom-atom pair distribution functions, gαβ(r), computed at zero density, it is shown that the shoulder present on the left hand side of the experimental g(r) is only due to the partial oxygen-deuterium distribution function, god(r).