Anchoring the potential energy surface of the cyclic water trimer

Six cyclic stationary points on the water trimer potential energy surface have been fully optimized at the MP2 level with the aug-cc-pVQZ basis set. In agreement with previous work, harmonic vibrational frequencies indicate that two structures are minima, three are transition states connecting minima on the surface while the remaining stationary point is a higher-order saddle point. The 1- and n-particle limits of the electronic energies of each of these six structures were estimated by systematically varying both the basis sets and theoretical methods. The former limit was approached with the cc-pVXZ and aug-cc-pVXZ families of basis sets (X=2-7) while MP2, CCSD(T), and BD(TQ) calculations helped examine the latter. Core correlation effects have also been assessed at the MP2 level with the cc-pCVXZ series of basis sets (X=2-5). These data have been combined to provide highly accurate relative energies and dissociation energies for these stationary points.