Development of a 3-body: many-body integrated fragmentation method for weakly bound clusters and application to water clusters (H2O)n=3-10, 16, 17

A 3-body: many-body integrated quantum mechanical (QM) fragmentation method for non-covalent clusters is introduced within the ONIOM formalism. The technique captures all 1-, 2-, and 3-body interactions with a high-level electronic structure method, while a less demanding low-level method is employed to recover 4-body and higher-order interactions. When systematically applied to 40 low-lying (H2O)(n) isomers ranging in size from n = 3 to 10, the CCSD(T):MP2 3-body: many-body fragmentation scheme deviates from the full CCSD(T) interaction energy by no more than 0.07 kcal mol(-1) (or < 0.01 kcal mol(-1) per water). The errors for this QM: QM method increase only slightly for various low-lying isomers of (H2O)(16) and (H2O)(17) (always within 0.13 kcal mol(-1) of the recently reported canonical CCSD(T)/aug-cc-pVTZ energies). The 3-body: many-body CCSD(T): MP2 procedure is also very efficient because the CCSD(T) computations only need to be performed on subsets of the cluster containing 1, 2, or 3 monomers, which in the current context means the largest CCSD(T) calculations are for 3 water molecules, regardless of the cluster size. (C) 2011 American Institute of Physics. [doi:10.1063/1.3609922]