Ab initio MO-VB study of water dimer

The equilibrium structure and binding energy of the water dimer system were determined by employing a general ab initio VB approach. Starting from the SCF-MI wavefunction, non-orthogonal virtual orbitals optimal for intermolecular correlation terms have been determined. BSSE is excluded in an a priori fashion and geometry relaxation effects are naturally taken into account. The equilibrium geometry corresponds to RO-O = 3.00 Angstrom, beta = 134.5 degrees and alpha = 2.5 degrees, in agreement with the experimental values. The donor OH bond results elongated by 0.002 Angstrom. The estimated equilibrium binding energy of the water dimer is -4.69 kcal/mol. Taking zero-point vibrational effects into account, the binding enthalpy is -3.1 kcal/mol, to be compared with the experimental estimate of -3.59 +/- 0.5 kcal/mol, determined from measurements of thermal conductivity of the vapour. (C) 1998 Elsevier Science B.V. All rights reserved.