The binding energies of the D2d and S4 water octamer isomers:: High-level electronic structure and empirical potential results

The MP2 complete basis set (CBS) limit for the binding energy of the two low-lying water octamer isomers of D-2d and S-4 symmetry is estimated at -72.7+/-0.4 kcal/mol using the family of augmented correlation-consistent orbital basis sets of double through quintuple zeta quality. The largest MP2 calculation with the augmented quintuple zeta (aug-cc-pV5Z) basis set produced binding energies of -73.70 (D-2d) and -73.67 kcal/mol (S-4). The effects of higher correlation, computed at the CCSD(T) level of theory, are estimated at <0.1 kcal/mol. The newly established MP2/CBS limit for the water octamer is reproduced quite accurately by the newly developed all atom polarizable, flexible interaction potential (TTM2-F). The TTM2-F binding energies of -73.21 (D-2d) and -73.24 kcal/mol (S-4) for the two isomers are just 0.5 kcal/mol (or 0.7%) larger than the MP2/CBS limit. (C) 2004 American Institute of Physics.