Figure 1. Potential energy surfaces (PES’s).

The PES’s of water dimers in the C_s, C_i, and C_2v geometries as a function of the oxygen-oxygen separation (R_OO) were obtained at various theoretical levels (solid curves). The dimer geometries were generated by varying R_OO, with all other geometrical parameters frozen at their MP2/aug-cc-pVTZ optimized values under the appropriate symmetry constraint. The accurate results from MP2/aug-cc-pVQZ (dashed curves) and CCSD(T)/aug-cc-pVQZ (dots) are superimposed. The binding energy at CCSD(T)/CBS is indicated as dashed black lines.