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 R O-O=3.00 Å, β=134.5° and α=2.5°, in agreement with the experimental values. The donor OH bond results elongated by 0.002 Å. 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