SCF,MP2, and CEPA-1 calculations on the OH ‥ O hydrogen bonded complexes (H2O)2 and (H2O-H2CO)

Counterpoise corrected ab initio calculations are reported for (H₂O)₂ and H₂O-H₂CO. Geometry searches were done in the moment-optimized basis DZP' at the SCF, MP2, and CEPA-1 levels of theory, followed by more accurate single-point calculations in basis ESPB, which includes bondfunctions to saturate the dispersion energy. The final equilibrium binding energies obtained are ?4.7 ±0.3 kcal/mol for a near-linear (H₂O)₂ structure and ?4.6 ±0.3 kcal/mol for a strongly bent HOH ‥ OCH₂ structure. The energy difference between these systems is much smaller than in all previous ab initio work. Cyclic (C_2h) and bifurcated (C_2v) transition structures for (H₂O)₂ are located at 1.0 ±0.1 kcal/mol and 1.9 ±0.3 kcal/mol above the global minimum, respectively. A new partitioning scheme is presented that rigorously partitions the MP2 correlation interaction energy in intra and intermolecular (dispersion) contributions. These terms are large (up to 2 kcal/mol) but of opposite sign for most geometries studied and hence their overall effect upon the final structures is relatively small. The relative merits of the MP2 and CEPA-1 approaches are discussed are discussed and it is concluded that for economical reasons MP2 is to be preferred, especially for larger systems.