HYDROGEN-BOND ENERGY OF THE WATER DIMER

Large scale air initio molecular orbital calculations on the binding e nergy of the water dimer have been performed. These calculations exten d the previous correlation consistent basis set work to include larger basis sets (up to 574 functions), and core/valence correlation effect s have now been included. The present work confirms the earlier estima te of -4.9 kcal/mol as the MP2(FC) basis set limit. Core/valence corre lation effects are found to increase the binding energy by similar to 0.05 kcal/mol. The best estimate of the electronic binding energy of t he water dimer is -5.0 +/- 0.1 kcal/mol. Correcting this value for zer o-point and temperature effects yields the value Delta H(375) = -3.2 /- 0.1 kcal/mol. This value is within the error limits of the best exp erimental estimate of -3.6 +/- 0.5 kcal/mol with the calculations favo ring the lower end of the experimental energy range. It should be usef ul to adopt the present estimate in empirical and semiempirical model potentials.