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?

Abstract

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 variational space approximation (RVS) due to Stevens and Fink.5 This enabled the quantification of the relative weights of each of the two individual ab initio second-order terms, polarization and charge-transfer (Epol and Ect, respectively) to nonadditivity. The SIBFA procedure was found to faithfully reproduce the ab initio results, both in terms of the total ΔE's and in terms of the separate nonadditive behaviors of its own Epol and Ect terms. It was also able to match very closely the results of the recent density functional theory computations of Lee et al.6 on cubic arrangements of water as occurring in ice. Thus, upon increasing n from 8 to 20, ΔE(SIBFA) was found to converge asymptotically toward a value of −11.5 kcal per molecule of water, close to the experimental binding energy of ice of −11.4. For n = 20 in this structure, the average dipole moment per water molecule was computed to be 2.74 D, itself very close to the value of 2.70 D in ice.