On the role of the basis set and electron correlation in the description of stacking interactions

Ab initio SCF and Moller-Plesset electron correlations, up to fourth-order calculations, have been performed for a very simple model system of stacking interactions-the water dimer with parallel and antiparallel dipole-dipole orientations. The performance of a variety of basis sets in evaluating the basis set superposition error and the interaction energy has been systematically examined. It is shown that for a proper description of the stacking interaction the diffuse sp-functions as well as the multiple sets of polarization functions are required. Additional calculations were carried out with popular semi-empirical methods (MNDO, AM1 and PM3). All of the applied semi-empirical methods greatly overestimate the interaction forces and give qualitatively incorrect results. Copyright (C) 1997 Elsevier Science Ltd.