Abstract
The present work attempts to assess and evaluate the performance of some new DFT methods in describing van der Waals (vdW) complexes that are characterized by the dominance of pure dispersion interactions. To achieve this goal, Argon dimers (Ar2) and trimers (Ar3) were investigated. As a reference calculation, the correlation interaction energy have been computed at the CCSD(T) level using the aug-correlated family of basis sets pVXZ (where X=2,3,4). Extrapolation to the CBS limit has been carried out and the behavior of the potential energy function has been analyzed and discussed. Correlation interaction energy has been computed at the MP2 and MP4 levels and compared to those calculated at the CCSD(T) method. Five new correlated DFT functionals, namely M06 and its long rang extension M06L, the B97-2 and its modified version B97-D which was deviseded for the dispersion interaction, and the PBEPBE and its correlated extension PBE0 methods have been used to compute the interaction energy in Ar2 and Ar3. The present work results indicate clearly that M06 and M06L did not only overestimate the equilibrium distance and depth but they also showed fluctuations in the potential energy curve near the minimum and along the dissociative arm. The B97-D and the PBE0 methods are much more reliable. However, these two later methods showed convergence problems when used to treat Ar3+; in addition to being extremely fast when compared to the CCSD(T) method extremely fast as compared to the CCSD(T). These features make them good candidate for investigating large vdW clusters. The BSSE has been estimated, analyzed and discussed. The relative stabilities of the excited states of Ar2 and Ar3 clusters together with those of the ionic species (Ar2+ and Ar3+)have been computed and analyzed.