Ab Initio Calculations of the Binding Energies of Small (H2O)n Clusters (n = 1…4)

The binding energies of the water dimer, trimer, and tetramer were calculated employing coupled‐cluster theory with singles, doubles, and non‐iterative triples (CCSD(TT)), and Møller‐Plesset second‐order perturbation theory (MP2) with terms linear in the interelectronic coordinates (MP2‐R12). Various atomic orbital (AO) basis sets were used to investigate the convergence of the binding energy towards the AO basis set limit. The calculations were performed at fixed geometries taken from the literature (which are geometries that were optimized at the MP2 level). Binding energies (De) of 20.9 kJ/mol (dimer), 69.2 kJ/mol (trimer), and 123.8 kJ/mol (tetramer) were obtained from MP2‐R12 calculations with a large 7s5p3d1f/5s3p1d AO basis set. These values are supposed to be very close (with errors of 2–5%) to the AO basis set limit. Binding energies obtained from CCSD(T) and MP2 calculations with the aug‐cc‐pVDZ basis set agreed within 3%. Hence, the MP2 level of calculation appeared to be appropriate to describe hydrogen‐bonding in small water clusters. All calculations were corrected for the basis set superposition error (BSSE) by the full counterpoise method.