Semiempirical model is developed for studying the electronic structure of the rare gas atom-halogen molecule systems. It is formulated in the frame of diatomics-in-molecule (DIM) approach and takes explicitly into account strong spin-orbit coupling pertinent to heavy halogen molecules. The consistent DLM scheme is realized for intermolecular interactions, whereas the description of valence electronic states of halogen molecule is more approximate being based on the asymptotic wave functions. The corresponding perturbation theory is also put forward. The model is applied to analysis of several features of the Ar ... I-2 van der Waals complex. First, the calculations on the spectroscopic constants of the B <-- X transition in the complex reveal the quantitative performance of the model. Second, mechanisms of nonadiabatic dynamics are examined. The results are qualitatively consistent with the current view on the Ar ... I-2 electronic predissociation and one-atom cage effect. Third, the prediction is made on the valence electronic spectrum of Ar ... I-2 complex. These examples demonstrate the reliability of the model.