HYDROGEN-BONDING DESCRIBED THROUGH DIATOMICS-IN-IONIC-SYSTEMS - THE HF DIMER

With the proper inclusion of ion-pair configurations? the diatomics-in -molecules formalism can be used to accurately describe hydrogen bondi ng. This is demonstrated for the well characterized prototype, the HF dimer, the structure and entire potential energy surface of which is r eproduced within its known accuracy: At the stationary points (potenti al minimum and saddle points) energies and bond lengths are reproduced with an accuracy of similar to 1%, and the soft hydrogen bond angles are determined to within similar to 5%. This is accomplished through a minimal basis Hamiltonian-19-dimensional matrix to describe the plana r complex-constructed with analytic fits to accurately known or determ ined pair potentials. The construct includes the H+F- ion-pair states of the HF monomer units. The three-body nature of the inductive ion-pa ir interactions with neutrals is preserved, in the spirit of diatomic- in-ionic-systems. Based on ab initio estimates, in the limited range o f interest, a Gaussian function describes the mixing between ionic and neutral states. The amplitude of this function is the only adjustable parameter in the model. The ionicity anisotropy and nonadditivity of interactions, responsible for the structure of the HF dimer, result na turally from mixing between ionic and neutral surfaces. (C) 1998 Ameri can Institute of Physics.