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 bonding. This is demonstrated for the well characterized prototype, the HF dimer, the structure and entire potential energy surface of which is reproduced within its known accuracy: At the stationary points (potential minimum and saddle points) energies and bond lengths are reproduced with an accuracy of ∼1%, and the soft hydrogen bond angles are determined to within ∼5%. This is accomplished through a minimal basis Hamiltonian—19-dimensional matrix to describe the planar complex—constructed with analytic fits to accurately known or determined pair potentials. The construct includes the H+F− ion-pair states of the HF monomer units. The three-body nature of the inductive ion-pair interactions with neutrals is preserved, in the spirit of diatomic-in-ionic-systems. Based on ab initio estimates, in the limited range of 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 naturally from mixing between ionic and neutral surfaces.