The density functional method with different basis sets was applied to the study of the highly efficient and low sensitive explosive 1,1-diamino-2,2-dinitroethylene in both gaseous dimer and its bulk state. The binding energies have been corrected for the basis set superposition errors. Four stable dimers (I, II, III, and IV) were located. The corrected binding energy of the most stable dimer IV is predicted to be -38.15 kJ/mol at the B3LYP/6-311++G** level. It was found that the structure of the most stable dimer is just the basic packing pattern in the wave-shaped layer of 1,1-diamino-2,2-dinitroethylene solid phase. Vibrational modes associated with the N-C-N rocking exhibits blueshifts with large intensities as the results of large dipole moment changes, whereas those assigned to the stretching of N-H, which is bound by another submolecule, exhibit large redshifts (over -21 cm(-1)) with respect to those of the monomer. The changes of Gibbs free energies (DeltaG) in the processes from the monomer to the dimers at 298.15 K are 16.46, 16.01, 11.85, and -1.78 kJ/mol for dimers I, II, III, and IV, respectively. Dimer IV can be spontaneously produced from the isolated monomer at room temperature. The calculated lattice energy is -105.81 kJ/mol, and this value decreases to -114.06 kJ/mol when a 50% correction of the basis set superposition error is adopted, which is in good agreement with the theoretical values already reported. The frontier bands are quite flat. Judging from the value of the band gap of 4.0 eV, it may be predicted that 1,1-diamino-2,2-dinitroethylene is an insulator. The frontier crystalline orbitals are formed by contributions from the C, N, and O atoms of the C-NO2 group, indicating that there exists a strong conjugation in the molecule and that the C-NO2 group is the most reactive part of it. The population of the C-NO2 bond is much less than those of the other bonds and the detonation may be initiated by the breakdown of this bond. (C) 2003 American Institute of Physics.