AB-INITIO STUDIES OF CYCLIC WATER CLUSTERS (H2O)(N), N=1-6 .3. 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. © 1995 American Institute of Physics.