Intermolecular interaction electron correlation effects

Electron correlation effects are known to be impx>rtant in systems with weak interactions. Studies of van der Waals interetctions have established the importance of using methods which scale linearly with the number of electrons[28] [29]. Of these methods, low-order many-body perturbation theory, in particular, second order theory, oflfers computational tractability combined with the ability to recover a significant firaction of the electron correlation energy. In the present work, second order many-body perturbation theory is used to account for correlation effects. Low order many-body perturbation theory has been used in accurate studies of intermolecular hydrogen bonding (see, for example, the work of Xantheas and Dunning[30]). 

Most popular in the ab initio calculation of intermolecular potentials is the so-called supermolecule method, because it allows the use of standard computer programs for electronic structure calculations. This method automatically includes all the electrostatic, penetration and exchange effects. If the calculations are performed at the SCF (self-consistent field) level the induction effects are included, too, but the dispersion energy is not. The latter, which is an intermolecular electron correlation effect, can be obtained by configuration interaction (Cl), coupled cluster (CC) calculations or many-body perturbation theory (MBPT). These calculations are all plagued... 

Fig. 8.12 the RDF by the full MM-MD method with the SPC/E water model as a reference since it was calibrated to correspond very well to the experimental results [62]. In the RDFs by the SD method (Fig. 8.12a), the first peak is found to exceed the reference (black curve) due to the inaccurate intermolecular interactions between one QM water molecule and close MM water ones in the solute. On the other hand, those by the NAM method with the BLYP and B3LYP methods (Fig. 8.12b) are found to be closer to the reference (black curve), while that with the HF method is lowered due to the neglect of the electronic correlation effect in the QM calculation and the second peaks cannot be accurately reproduced by the NAM method. In this study, we have included four water molecules in the QM region as the first solvation shell, and this discrepancy is probably due to the inaccurate... 

The SCF-MI BSSE free method does not take into account dispersion forces, connected to electronic intermolecular correlation effects. By using the SCF-MI wave function as a starting point, however, a non orthogonal BSSE free Cl procedure can be developed. This approach was applied to compute intermolecular interactions in water dimer and trimer the resulting ab initio values were used to generate a new NCC-like potential (Niesar et al, 1990). Molecular dynamics simulation of liquid water were performed and satisfactory results obtained (Raimondi et al, 1997). 

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. 

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