The structure, tautomerism, and vibrational spectra of the highly explosive 3-amino-5-nitro-1,2,4-triazole (ANTA) molecule were studied by ab initio molecular orbital calculations at the Hartree-Fock and second-order (MP2) and fourth-order (MP4) Moller-Plesset levels and by density functional theory (DFT) both in gas phase and in solution. It was found that in the gas phase the most stable tautomer is 2H-ANTA (3-amino-5-nitro-1,2,4-2H-triazole) at the HF level, while at MP2, MP4, and DFT levels the most stable tautomer is 1H-ANTA (3-amino-5-nitro-1,2,4-1H-triazole). For the 2H-ANTA tautomer, the calculated MP2 and DFT structures agree well with the experimental X-ray structures, but with the twisting of the nitro and amino groups much larger than in the solid state. The predicted IR spectra are given for all tautomers. The calculated fundamental vibrational frequencies at the DFT level generally compare well with the MP2 results. In the case of 2H-ANTA, the C-NO2, C-NH2, and N-H bond dissociation energies were estimated to be 67.1, 112.8, and 97.2 kcal/mol, respectively. On the basis of the structural and spectroscopic results calculated at the MP2 level, a classical force field for gas-phase 2H-ANTA was developed. The effects of environment polarity on the conformations and energetics of 1H-ANTA and 2H-ANTA tautomers were studied by performing calculations at the DFT level within the Onsager continuum solvation model. The results obtained for the dielectric constants epsilon = 4.8, 18.5, and 78.4 suggest that in a polar solvent the most stable tautomer is 2H-ANTA, in agreement with the experimental studies.