ENERGY ANALYSIS ON SMALL TO MEDIUM-SIZED H-BONDED COMPLEXES

Dimers (water-methanol, guanine-cytosine) as well as trimers (methanol -water-imidazole, formamide-methylformate-formamide), are studied as H -bonded complexes of increasing complexity. All the investigated confo rmations have been fully optimized. In particular, it is the first tim e that all the intra- and intermolecular parameters of the guanine-cyt osine complex are left variable. In minimal basis sets, the planar con formation has been found to be a first-order critical point. The minim al basis set MINI-1 has been adapted to provide nearly planar amides. The stability of the complexes is accounted for by four energy compone nts of the same order: the first-order term (electrostatic + exchange) , the polarization, the charge transfer and the correlation terms. In the case of the studied trimers, the energy components, apart from the electrostatic one, have been found to be nearly additive.