BSSE free MCSCF method for strong hydrogen bonds: investigation of H2O-HCl and NH3-HCl complexes

A Multi-Configuration extension of the Self Consistent Field for Molecular Interaction method – MCSCF-MI – is presented. Applications to the study of hydrogen bonded systems are reported. Potential energy, optimal geometry and the characteristic H-Cl stretching in the H2O-HCl and NH3-HCl complexes were examined. BSSE, which represents a major problem in the study of the interaction potentials of loosely bonded fragments, is avoided by the a priori SCF-MI technique. Equilibrium geometries, binding energies, and vibrational constants computed by both SCF-MI and MCSCF-MI methods turned out more stable than the corresponding standard SCF calculations upon variation of the quality of the basis set employed. In particular, the proton transfer process was examined. All the calculations predicted that the minimum corresponds to hydrogen bonded structures. In the case of NH3-HCl good agreement with recent extensive MO-CI and DFT studies was establish