A diatomics-in-molecules case study on the system Be+HF→BeF+H. I. Bonding models and the use of valence bond information

Abstract

DIM calculations on the ground state potential energy surface for Be+HF→BeF+H are approached on the basis of a variety of bonding models, involving from three up to 18 structures. A considerable input of diatomic fragment information is required. To this end two sets of ab initio valence bond calculations have been carried out on the fragments, the first restricted to just those structures contributing to the largest DIM model and the second containing many additional fragment structures. VB calculations at the first level were extended to compute the full triatomic potential surface and this is found to agree qualitatively fairly closely with that from a DIM model based on the same structures and fragment curves. A preliminary survey highlights two practical difficulties in using data from large VB calculations to support smaller DIM models (a) in numerical interpolation of coupling constants and (b) in identification of the appropriate fragment curves to use. It is also shown that inclusion of additional structures in a DIM calculation may in certain examples lead inevitably to a raising of the calculated interaction energy.