Rationalizing aggregate structures with orbital contributions to the exchange-repulsion energy

It is shown that repulsive interactions have a crucial influence on the structure of prototypical non-covalently bonded systems. To explain this, we propose a molecular orbital based model for the exchange-repulsion contribution to the total interaction energy. As a central result, our model shows that energetically preferred aggregate structures exhibit reduced exchange repulsion, which can be deduced from the nodal structure of certain occupied orbitals. In this way, the directionality of halogen bonds and the preferred arrangements in pi-aggregates are explained using the Cl2-He, benzene-benzene, and benzene-hexafluorobenzene systems as examples, where commonly applied electrostatic models fail.