Effects of monomer geometry and basis set saturation on computed depth of water dimer potential

The interaction energy for the water dimer has been calculated using supermolecular many-body perturbation theory (MBPT) at the fourth-order level, the coupled clusters method with single, double, and noniterative triple excitations [CCSD(T)], and the symmetry-adapted perturbation theory (SAPT). We argue that the appropriate monomer geometry in such calculations has to be the average geometry of the ground vibrational state rather than the customarily used equilibrium geometry. The use of the former instead of the latter geometry increases the dimer binding energy by about 0.12 kcal/mol in the van der Waals minimum region almost independently of the method employed. Our largest basis set with a balanced account of the intramonomer correlation and dispersion effects gives interaction energy at the second-order MBPT level which is 0.03 kcal/mol lower than the best previous literature value. The final depth at the minimum obtained using SAPT is 5.05 kcal/mol, while the commonly accepted empirical depth is 5.4 +/- 0.7 kcal/mol. Taking into account the fact that the empirical result contains a theoretical zero-point energy which is probably overestimated by a few tenths of kcal/mol, our value of the potential depth believed to be accurate to within 0.1 kcal/mol is in a better agreement with experiment than the results of recent large-scale ab initio calculations. The optimized dimer geometry agrees to within 0.001 Angstrom and 2 degrees with the experimental geometry from microwave measurements. (C) 1996 American Institute of Physics.