Clusters, intermolecular interaction energy

With the success of these calculations for isolated molecules, we began a systematic series of supermolecule calculations. As discussed previously, these are ab initio molecular orbital calculations over a cluster of nuclear centers representing two or more molecules. Self-consistent field calculations include all the electrostatic, penetration, exchange, and induction portions of the intermolecular interaction energy, but do not treat the dispersion effects which can be treated by the post Hartree-Fock techniques for electron correlation [91]. The major problems of basis set superposition errors (BSSE) [82] are primarily associated with the calculation of the energy. 

After generating optimized clusters, the intermolecular interaction energies between pair molecules a and / ( ) in the cluster were calculated using the supermolecular approach ... 

The results of the decomposition of the intermolecular interaction energy of cluster M are not qualitatively different from those found for the same cluster M in vacuo. There are quantitative changes, often of not negligible entity and that correspond, in general, to a damping of the effects. 

Table 1 Intermolecular interaction energy (E ) for some small clusters (kcal/mol) ...

The Dilute Mixture of Water in Methane. When one molecule of water is surrounded by methane molecules, the molecule of water behaves like a regular nonpolar molecule (see Figure 2, where one of the typical minimized clusters 1 (water) 10 (methane) is presented). The average intermolecular distance and interaction energy between a water molecule and the nearest neighbors methane molecules in the clusters H20 "(CH4)io are listed in Table 5. 

Ab initio quantum mechanical methods were recently applied to the analysis of large clusters formed of one solute molecule and several molecules of solvent for water/methane mixtures. It was shown that they can provide information regarding the interaction energies and intermolecular distances between the molecules of methane and water. The obtained results were compared to the available experimental and molecular simulations regarding condensed mixtures, and agreement was found. A similar methodology of calculations will be used in the present paper as well. 

The results listed in Tables 2—4 show that the interaction energies and the intermolecular distances between the molecules of water and methanol are quite different in the CH40 (H20)i2 and H20 "(CH40)io clusters. This difference reflects the fact that the two kinds of clusters represent two different physical systems. Indeed, the two extreme cases at mole fractions of Xi... 

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