Abstract
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.
Highlights
The interest in cluster research has grown enormously during the last decade, due to its intermediate position; clusters represent the bridge to the understanding of the transition from the gas-phase to the condensed phase
The question of what is the appropriate method to be used to evaluate vdw interactions, still exists. This is due to three main reasons concerning the size and flexibility of the basis set, electron-correlation and basis set superposition error
Extension to relatively large molecular system is governed by basis set limitations due to computational demands
Summary
The interest in cluster research has grown enormously during the last decade, due to its intermediate position; clusters represent the bridge to the understanding of the transition from the gas-phase to the condensed phase. Dispersion interactions are either isotropic as in the case of rare-gases or anisotropic in nature as in the case of HF, CO. In the former case, empirical fitting of experimental data has successfully yielded both interaction energies and geometries, whereas in the case of anisotropic interaction, a quantitative description is currently beyond experimental limits. The question of what is the appropriate method to be used to evaluate vdw interactions, still exists. This is due to three main reasons concerning the size and flexibility of the basis set, electron-correlation and basis set superposition error. The Ar dimer is one of the most interesting vdw molecules and is the subject of several experimental [11,12,13,14,15] and theoretical investigations [2-
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