Benchmark Structures and Harmonic Vibrational Frequencies Near the CCSD(T) Complete Basis Set Limit for Small Water Clusters: (H₂O)_{<i>n</i> = 2, 3, 4, 5, 6}

A series of (H₂O)_<i>n</i> clusters ranging from the dimer to the hexamer have been characterized with the CCSD­(T) and the 2-body:Many-body CCSD­(T):MP2 methods near the complete basis set (CBS) limit to generate benchmark-quality optimized structures and harmonic vibrational frequencies for these important systems. Quadruple-ζ correlation-consistent basis sets that augment the O atoms with diffuse functions have been employed in the analytic computation of harmonic vibrational frequencies for the global minima of the dimer, trimer, tetramer, and pentamer as well as the ring, book, cage, and prism isomers of the hexamer. Prior calibration [<i>J. Chem. Phys.</i> <b>2013</b>, <i>139</i>, 184113 and <i>J. Chem. Theory Comput.</i> <b>2014</b>, <i>10</i>, 5426] suggests that harmonic frequencies computed with this approach will lie within a few cm^–1 of the canonical CCSD­(T) CBS limit. These data are used as reference values to gauge the performance of harmonic frequencies obtained with other ab initio methods (e.g., LCCSD­(T) and MP2) and water potentials (e.g., TTM3-F and WHBB). This comparison reveals that it is far more challenging to converge harmonic vibrational frequencies for the bound OH stretching modes in these (H₂O)_<i>n</i> clusters to the CCSD­(T) CBS limit than the free OH stretches, the <i>n</i> intramonomer HOH bending modes and even the 6<i>n</i> – 6 intermonomer modes. Deviations associated with the bound OH stretching harmonic frequencies increase rapidly with the size of the cluster for all methods and potentials examined, as do the corresponding frequency shifts relative to the monomer OH stretches