Theoretical investigation of the dispersion interaction in argon dimer and trimer

The ultimate aim of the present work is to establish an acceptable level of computation for the van der waals (vdw) complexes that is able to pick up appreciable amount of dispersion interaction energy, reproduce the equilibrium separation within the acceptable limits and at the same time cost and time effective. In order to reach this aim vdw clusters where pure isotropic dispersion interaction occur, namely, Ar dimer and trime were investigated. Computations using different basis sets and at different levels of theory have been carried out. Three basis sets, namely, the 6-31++G**, the 6-311++G** and the aug-cc-pvdz basis set, were found superior to all other basis sets used. The high performance and relative small CPU time of the 6-31++G** basis set make it a good candidate for use with large vdw clusters, especially those of interest in biology. Three compound methods were applied in the present work, namely G1, G2 and G2 Moller-Plesset (MP2) and the complete basis set method, CBS-Q. These methods failed to detect the attraction dispersion interaction in the dimer. The predicted repulsive interaction seems dominant in all these methods. Some of the recently developed Density Functional Theory (DFT) methods that were parameterized to account for the dispersion interaction were also evaluated in the present work. Results come to the conclusion that, in contrast to the claims made, state-of-the-art Density Functional Theory methods are incapable of accounting for dispersion effects in a quantitative way, although these methods predict correctly the inter-atomic separations and are thus considered a real improvement over the conventional methods. BS-SE has been computed, analyzed and discussed.