  For the prototypical diatomic-molecule - diatomic molecule interactions H2-HX
and H2-X2, where X = F, Cl, Br, quantum-chemical ab initio calculations are
carried out on grids of the configuration space, which permit a
spherical-harmonics representation of the potential energy surfaces (PESs).
Dimer geometries are considered for sets of representative leading
configurations, and the PESs are analyzed in terms of isotropic and anisotropic
contributions. The leading configurations are individuated by selecting a
minimal set of mutual orientations of molecules needed to build the
spherical-harmonic expansion on geometrical and symmetry grounds. The terms of
the PESs corresponding to repulsive and bonding dimer geometries and the
averaged isotropic term, for each pair of interacting molecules, are compared
with representations in terms of a potential function proposed by Pirani et al.
(see Chem. Phys. Lett. 2004, 394, 37-44 and references therein). Connections of
the involved parameters with molecular properties provide insight into the
nature of the interactions.
