Hydrogen bond energy of the water dimer

Large scale air initio molecular orbital calculations on the binding energy of the water dimer have been performed. These calculations extend the previous correlation consistent basis set work to include larger basis sets (up to 574 functions), and core/valence correlation effects have now been included. The present work confirms the earlier estimate of -4.9 kcal/mol as the MP2(FC) basis set limit. Core/valence correlation effects are found to increase the binding energy by similar to 0.05 kcal/mol. The best estimate of the electronic binding energy of the water dimer is -5.0 +/- 0.1 kcal/mol. Correcting this value for zero-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 experimental estimate of -3.6 +/- 0.5 kcal/mol with the calculations favoring the lower end of the experimental energy range. It should be useful to adopt the present estimate in empirical and semiempirical model potentials.