Hartree-Fock calculations of a cyclotrimethylene trinitramine C3H6N6O6 (RDX) crystal containing vacancy dimers were performed using a periodic defect model and large supercells. Positive binding energies of the divacancies indicate that single vacancies attract each other in RDX crystals, thus forming defect complexes. One- and two-step processes of vacancy pair formation have been simulated. The density of states and band structure analysis show that the defect pairs narrow the fundamental optical gap. Mutual orientation within vacancy dimers considerably effects the electronic properties. A detailed comparison of perfect RDX, that containing a single vacancy, and that with a divacancy is presented. The role of these defects in the "hot spot" formation and shock initiation of explosions is discussed.