The three reactions NH3++NH3-->NH2+NH4+ (proton transfer), NH3++NH3-->NH4++NH2 (atom transfer) and NH3++NH3-->NH3+NH3+ (charge transfer) are studied in an ab initio framework. All geometry optimizations are carried out at the MP2 level, and a SDCl(TQ) calculation is performed at the optimized geometry. For the charge transfer reaction, the energy is calculated as a function of the N-N internuclear distance. The intermediate complex is found to have D-3d symmetry. The geometry of the NH3+NH3+ system is optimized for each value of the N-N distance. For the proton transfer reaction, the energy is calculated as a function of two variables which are the two N-H internuclear distances of the central part N-H-N of the complex. For each N-H-N configuration, other coordinates of the system are completely optimized. This approach shows that the atom transfer reaction can be interpreted as a charge transfer process followed by a proton transfer. The influence of the vibrational excitation of the NH3+ reagent on the reaction is discussed.