Molecular orbital treatments of hydrogen bonded systems. 2. Dimers of water and HCN

Calculations were performed to investigate the reliabilities of the CNDO/2, PRDDO, and MNDO approximate molecular orbital methods. Systems selected for study included the linear, cyclic, and bifurcated dimers of water as well as the linear and cyclic dimers of HCN. The PRDDO method was found to provide the most consistently accurate reproduction of ab initio and experimental data. CNDO/2 performed fairly well in a number of cases but yielded extremely poor results for the cyclic dimers of both H₂O and HCN. Hydrogen bond strengths were consistently underestimated by MNDO which also furnished erroneously large intermolecular separations. In addition, MNDO calculations indicate the bifurcated water dimer to be most stable in contrast to other quantum mechanical and experimental information.See Ref. [1] for Paper 1 of this series.