The equilibrium structures and spectral properties of neptunyl nitrate complexes NpO2(NO3)(n)(q) (n = 1-3, q = +1, 0, -1) and hydrated dinitrate complexes NpO2(NO3)(2)(H2O)(m) (m = 1-4) have been studied systematically by carrying out DFT calculations. It is found that the neptunyl ion tends to bind to the nitrate ion in a bidentate fashion, and the 6 coordinate dinitrate in the m=2 complex is dominant in these hydrated neptunyl dinitrate complexes. This conclusion is strengthen by the good agreement between the calculated structural parameters, IR spectra of NpO2(NO3)(2)(H2O)(2) and the latest experimental data. (C) 2015 Elsevier B.V. All rights reserved.