Variation-perturbation treatment of the hydrogen bond between water molecules

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Abstract
The hydrogen-bond energy of two water molecules has been calculated as a sum of the electrostatic, exchange, induction and dispersion contributions, neglecting the electron correlation within the free monomers. The last two contributions have been evaluated by applying a variation-perturbation procedure and making use of an extensive basis set of contracted gaussian functions. It has been shown that the sum of the electrostatic, exchange and induction energies is very close to the binding energy obtained within the SCF scheme. The dispersion contribution to the hydrogen-bond energy amounts to about 2 kcal/mole and causes substantial reduction of the equilibrium distance of the oxygen atoms. The minimum of the total energy is attained at 2·86 Å and its depth is equal to 5·8 kcal/mole. These values are consistent with the experimental results.