Improved intermolecular water potential from global geometry optimization of small water clusters using local MP2

Global geometry optimizations of water pentamer and hexamer clusters were carried out using a model potential from the NEMO family [P.-O. Astrand, A. Wallqvist, G. Karlstrom, J. Chem. Phys. 100 (1994) 3726], which is simultaneously improved by minimizing the interaction energy deviations from an ab initio reference tin our case local second-order Moller-Plesset perturbation theory, LMP2) at corresponding geometries. This procedure not only yields the most important local land global) minima of these clusters at the LMP2 level, but also an improved parameter set for the NEMO potential that, in contrast to the original NEMO potential, reproduces these structures and their energy ordering faithfully. The use of local MP2 ensures that the optimized structures are virtually free of basis set superposition errors. (C) 1998 Elsevier Science B.V. All rights reserved.