This study focuses on elucidating the stable forms of a new energetic material that is a member of the class of insensitive munitions (IM), 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), including its tautomers, and anions. The geometry and properties of all compounds were calculated using density functional theory (M06-2X) and MP2 quantum chemical approaches. Calculations were carried out in the gas phase and in aqueous solution. Chemical stability of these compounds was evaluated in terms of the Gibbs free energy change. Two different solvation models were applied (CPCM and PCM). Calculations showed that overall differences in the results obtained using these two solvation models are negligible for all compounds considered. All possible NTO tautomers were examined and the results are in good agreement with previous studies performed in the gas phase. The stability order was revealed to be slightly dependent on the method applied. In order to estimate acidic properties of NTO, anions of several NTO tautomers were analyzed. In addition, pK a values were calculated using different approaches. As compared with available experimental data it was found that the conductor-like screening model for real solvents approach leads to more accurate estimation of the pK a value than the CPCM and PCM approaches. The pK a value calculated using PCM and CPCM data showed large errors; however, it was proven that the pattern of deprotonation energy was correctly estimated.