A bond-bond description of the intermolecular interaction energy: the case of weakly bound N(2)-H(2) and N(2)-N(2) complexes.

The atom-bond pairwise additive approach, recently introduced by us to describe the potential energy surface for atom-molecule cases, is extended here for the first time to molecule-molecule systems. The idea is to decompose the van der Waals interaction energy into bond-bond pair contributions. This must be considered an improvement with respect to the familiar atom-atom pairwise additive representation since, still using a simple formulation, it indirectly accounts for three body effects. Such an approach also allows to include, in a straightforward way, the effect of the bond length on the intermolecular interaction energy. Cases of study are the weakly bound complexes involving the H(2) and N(2) molecules, namely N(2)-H(2) and N(2)-N(2), here described as a single bond-bond pair. For both systems ab initio calculations and experimental molecular beam scattering data, as well as second virial coefficients, have been employed to test the accuracy of the chosen representation of the interaction and to improve the obtained potential energy surfaces. The results of this work are important also for the generalization to the cases involving molecular ions and polyatomic molecules.