Ab initio studies of cyclic water clusters (H2O)n, n=1–6. III. Comparison of density functional with MP2 results

The optimal structures and harmonic vibrational frequencies of ring water clusters (H2O)n, n=1–6 are computed using density functional theory (DFT). The exchange functionals of Slater (S), Becke (B), the correlation functionals of Lee–Yang–Parr (LYP), Vosko–Wilk–Nusair (VWN), Perdew’s local (PL), and gradient-corrected (P86) as well as their combinations are used to perform benchmark calculations on the water monomer and dimer. We use the augmented correlation-consistent polarized valence orbital basis set of double (aug-cc-pVDZ) and triple zeta quality (aug-cc-pVTZ) in order to compare the DFT with earlier MP2 results J. Chem. Phys. 99, 8774 (1993); 100, 7523 (1994). Better overall agreement with the MP2 and experimental results for the water monomer and dimer is observed for the B–LYP and B–P86 functional combinations. The optimal structures, harmonic vibrational frequencies, and energetics of the clusters trimer through hexamer are computed at the B–LYP/aug-cc-pVDZ level of theory. This functional combination yields structures, energetics, and trends with cluster size that are in good agreement with the corresponding MP2 results.