ABINITIO CALCULATION OF THE 2ND VIRIAL-COEFFICIENT OF NEON AND THE POTENTIAL-ENERGY CURVE OF NE2

For the ab initio calculation of properties of dense gases and liquids accurate potential energy curves are needed. Here we describe a method to obtain an "optimal" curve for a given number of ab initio calculations. Strictly speaking, we give a recipe, with which interatomic distances calculations should be performed to yield the second virial coefficient with a minimal loss in accuracy due to the limited number of points. With this technique we obtain the virial coefficient from about 15 points essentially without loss in accuracy. Experimental data for a direct test of the quality of the potential energy curve are scarce. For the neon dimer we have calculated the potential well depth, the equilibrium distance and the vibrational frequency for such a comparison with experiment. An indirect indication of the quality of the curve is given by the calculation of the second virial coefficient at different temperatures. The best calculations presented here use extended basis sets, correct for the basis set superposition error by the counterpoise method, and include electron correlation using Moller-Plesset perturbation theory on the MP4(SDTQ) level.