MESQUAC: mixed electrostatic–quantum chemical approach to the description of large complexes

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Abstract
A mixed electrostatic–quantum chemical description of large complex systems is proposed. The central ion and one of the ligands are treated by means of a LCAO-MO-SCF procedure while further ligands are handled as a “perturbation” in the form of electrostatic point charges distributed according to population analysis of the ligand atoms in the 1 : 1 complex. The approach has been tested by means of calculation on aquocomplexes of Li, Na, K, Mg and Ca ions with coordination numbers from 2 to 6. For Li, Na and Mg a second hydration sphere has also been considered. The results for all calculations on the complexes of Li, Na and Mg are of the same quality as those from full ab initio calculations, the computing time being greatly reduced. For the larger ions (K, Ca) less satisfactory results were obtained because the use of a small basis set led to an overemphasized charge transfer towards the central ion in the 1 : 1 complex. This failure could be corrected, however, by neglecting this charge transfer effect in the choice of the point charges representing the ligands.

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