Model, Multiply Hydrogen-Bonded Water Oligomers (N = 3−20). How Closely Can a Separable, ab Initio-Grounded Molecular Mechanics Procedure Reproduce the Results of Supermolecule Quantum Chemical Computations?

Several high-level quantum chemical calculations have highlighted the prominent weight of nonadditivity within the total stabilization energy of multiply hydrogen-bonded complexes, such as exemplified by water oligomers (reviewed in ref 1). We have evaluated the extent to which the SIBFA (sum of interactions between fragments ab initio computed) molecular mechanics procedure2-4 could account for nonadditivity in such complexes. This takes advantage of the separability of the energy expression into five distinct components, each of which have inherent anisotropic features. For that purpose, we have considered several representative water oligomers encompassing from n = 3 to n = 20 molecules, in cyclic and acyclic, as well as, for n = 6 and beyond, tridimensional cubic arrangements. Single-point ab initio SCF and MP2 supermolecular energy computations were performed in the energy-minimized structures. A decomposition of the SCF intermolecular interaction energy was done, for n = 3−6, using the restricted va...