Rubidium nitrate has different crystalline modifications (phases I-IV), and the most interesting among them is the orientationally disordered phase III which is the most conducting. We used classical molecular dynamics simulation to study the mechanism of ion migration and the role of the orientational disordering in the high ionic conductivity. In our preliminary simulation (N.F. Uvarov et al., Solid State Ionics. 188 (2011) 78-82) the point defect formation energy was calculated, and it was found that phase III demonstrates lower values of the energy than phases II and IV. It was suggested that the conductivity in the rubidium nitrate can be limited by the defect formation rather than the cationic migration process. In this paper we have performed additional simulations and also calculated the activation energy of cationic migration for phases III and IV. This energy is less than the energy of the defect formation by one order of magnitude in both phases. It confirms our suggestion that the main reason for the high conductivity of phase III is the lower energy of the defect formation. (C) 2013 Elsevier B.V. All rights reserved.