A SEMIEMPIRICAL SCHEME FOR GENERALIZED VALENCE-BOND CALCULATIONS ON WATER COMPLEXES

A method is introduced for carrying out semi-empirical energy calculat ions using a localized valence-bond wavefunction. Chemical bonds are d escribed by correlated electron pairs, with interactions between pairs treated iteratively with a classical approximation at large distance. The model consists of a minimal basis of localized hybrid valence orb itals with semi-empirical approximation for the electron integrals and core repulsions. Parameters are determined for oxygen and hydrogen wh ich are fit to the MP2/6-31G* potential surfaces of H2O and (H2O)(2) with 0.5 kcal/mol accuracy. Results are shown for other neutral and ch arged water clusters, including a water chain of up to 200 H2O molecul es for which the calculation shows O(N-1.5) scaling with size. (C) 199 7 American Institute of Physics.