CCSD(T) Complete Basis Set Limit Relative Energies for Low-Lying Water Hexamer Structures

MP2 and CCSD(T) complete basis set (CBS) limit relative electronic energies (Delta E-e) have been determined for eight low-lying structures of the water hexamer by combining explicitly correlated MP2-R12 computations with higher-order correlation corrections from CCSD(T) calculations. Higher-order correlation effects are quite substantial and increase Delta E-e by at least +0.19 kcal mol(-1) and as much as +0.59 kcal mol(-1). The effects from zero-point vibrational energy (ZPVE) have been assessed from unscaled harmonic vibrational frequencies computed at the MP2 level with a correlation consistent triple-zeta basis set (cc-pVTZ for H and aug-cc-pVTZ for O). ZPVE effects are even more significant than higher-order correlation effects and are uniformly negative, decreasing the relative energies by -0.16 kcal mol(-1) to -1.61 kcal mol(-1). Although it has been widely accepted that the cage becomes the lowest-energy structure after ZPVE effects are included [Nature 1996, 381, 501-503], the prism is consistently the most stable structure in this work, lying 0.06 kcal mol(-1) below the nearly isoenergetic cage isomer at the electronic MP2 CBS limit, 0.25 kcal mol(-1) below at the electronic CCSD(T) CBS limit, and 0.09 kcal mol(-1) below at the harmonic ZPVE corrected CCSD(T) CBS limit. Moreover, application of any uniform scaling factor less than unity to correct for anharmonicity further stabilizes the prism and increases the relative energies of the other structures.