A theoretical technique in which dispersion effects, calculated via long-range perturbation theory, are added to ab initio Hartree-Fock calculations is applied to a study of homogeneous and mixed dimers of benzene and s-tetrazine. The most stable geometry of benzene dimer is found to be of perpendicular or "T-shaped" type. Two stable structures are identified for tetrazine dimer; a coplanar side-by-side arrangement and a perpendicular one. The most stable mixed dimer of benzene with tetrazine contains a stacked parallel arrangement of the two molecules. All results are in good agreement with experimental observation of gas-phase dimers and crystal structures. It is shown that many of the results can be predicted on the basis of electrostatic effects involving quadrupole-quadrupole interactions.