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 (C2h) and bifurcated (C2v) 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. Copyright © 1990 John Wiley & Sons, Inc.