Abstract
Within the context of extended geminal models the concepts of charge centroids and charge ellipsoids of the geminal one-electron densities, and an energy decomposition of the intermolecular potential, are introduced as tools of analysis. The intermolecular potential can within this framework be written as a sum of the distortion energies of the subsystems and the interaction energies between the distorted subsystems. The interaction energy is further partitioned into a Coulombic, exchange and correlation contribution. Three classes of complexes are studied: hydrogen bonded systems (HF)2, H2OHF, (H2O)2; strongly bonded electron donor-acceptor (EDA) complexes: BH3NH3, BH3CO; and weakly bonded EDA complexes: F2NH3, Cl2NH3 and ClFNH3. The main results of the calculations, using basis sets consisting of [9s, 6p, 2d](Cl), [7s, 4p, 2d] (B, N, O, F), [4s, 2p](H) contracted Gaussian-type functions, and the numerical models EXRHF3 and EXGEM7, are as follows. The bonding in these complexes is essentially due to a lone pair of the donor subsystems approaching the “vacant” space in the vicinity of a nucleus of the acceptor system. The interaction energy is therefore dominated by the Coulombic term. However, the sum of the distortion terms is larger than the magnitude of the Coulombic term. Hence, the exchange and correlation terms give a substantial contribution to the intermolecular potential. If the components of the decomposition of the potential are resealed by using the magnitude of the interaction energy as the energy unit, a remarkable similarity between the three classes of complexes is disclosed.
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