A new variational coupled-electron pair approach to the intermolecular interaction calculation in the framework of the valence bond theory: The case of the water dimer system

A general nonorthogonal coupled-electron pair approach based on the interme diate optimization of virtual orbitals is presented. The resulting procedur e, similar to the independent electron pair approximation scheme, is develo ped in the framework of the valence bond (VB) theory, where the effect of t he overlap is directly taken into account. Nonorthogonal virtual orbitals o ptimal for intermolecular correlation effects were determined starting from the self-consistent field for molecular interaction wave function. These w ere used in the context of a general ab initio variational multistructure V B wave function consisting of double excitations arising from simultaneous single excitations localized on each monomer. The basis set superposition e rror is excluded in an a priori fashion and geometry relaxation effects are naturally taken into account. As an application example, the equilibrium s tructure and binding energy of the water dimer system were determined. The equilibrium oxygen-oxygen distance results to be 2.954 Angstrom, in good ag reement with the experimental values (2.946 or 2.952 Angstrom) corrected fo r anharmonicity of the dimer vibrations. The estimated equilibrium interact ion energy is -5.02 kcal/mol, thus comparing favorably with the experimenta l value of -5.44 +/- 0.7 kcal/mol. Taking zero-point vibrational effects in to account, the calculated binding enthalpy is -3.22 kcal/mol, in accordanc e with the experimental estimate of -3.59 +/- 0.5 kcal/mol, determined from measures of thermal conductivity of the vapor. The importance of employing basis sets that include diffuse polarization functions in correlated calcu lations on hydrogen-bonded systems is confirmed. (C) 1999 American Institut e of Physics. [S0021-9606(99)30637-1].