A hybrid quantum mechanical diatomics-in-molecules (QM/DIM) method is formulated that aims to describe chemical reactions in an environment within the framework of a discrete (or cluster) approach. Starting from the conventional DIM formalism, first-order intermolecular perturbation theory is applied to calculate interactions between reacting particles and environmental molecules, and to introduce ab initio QM energies for the central system. In this approach no boundary problems appear when combining two parts of the entire system. The pairwise contributions to the interaction energy come from the true potential curves of the fragments. A proper treatment of excited electronic states is also provided in this approach. As a first application, we compute potential curves for the dissociation reaction of a single hydrogen fluoride molecule surrounded by a selected number of solvent HF molecules.