Proton transfer in gaseous ammonium dinitramide (ADN) clusters up to (ADN)(2) is studied by using density-functional theory. Proton transfer between the hydrogen dinitramide and ammonia units does not occur in the ADN monomer, rather the ammonia-hydrogen dinitramide complex is stabilized by strong hydrogen bonding. However, proton transfer between hydrogen dinitramide and ammonia is observed in the ADN dimer [NH3HN(NO2)(2)](2), ADN solvated with a single ammonia molecule [NH3NH(NO2)(2)]NH3, and ADN solvated with a hydrogen dinitramide molecule [NH3HN(NO2)(2)]HN(NO2)(2). Structural changes in the complexes relative to the free molecules and the binding energies of the clusters are given. Using population analysis, the total electrostatic interaction energy in each cluster is calculated. The electrostatic energy is a measure that distinguishes between the ionic or hydrogen-bonded nature of the clusters. Some implications of proton transfer in ADN clusters on the decomposition mechanism of ADN are discussed. (C) 2003 American Institute of Physics.