The molecular geometries, electronic structures, IR spectra and thermodynamic properties in 298～1000K range of α(γ), β, and ε- conformations of hexanitrohexaazaisowurtzitane (HNIW) have been calculated using ab initio and density functional theory (DFT) methods at HF/6-31G^* and B3LYP/6-31G^* level, respectively. The results obtained from the two methods have been carefully compared with each other and with the experimental data. The optimized parameters of geometry are in good accordance with the experimental values. Compared with the other bonds in HNIW, the bond lengths of N- N- are longer and Mulliken population of N-N is smaller, which means that the N-N may be the initial bondin pyrolysis and explosion. The obtained IR spectra are also in good accordance with the experimental results and the average absorlute difference is less than 45cm^-1. The thermodynamic stability order [ε>α(γ)>β] predicted from frontier molecular orbital energies and their gaps is the same as that measured from experiments.

Key words: MOLECULAR STRUCTURE, ELECTRONIC STRUCTURE, INFRARED SPECTROPHOTOMETRY, THERMODYNAMIC PROPERTIES, AB INITIO CALCULATION, POLYCYCLO CAGE COMPOUND, NITRO-AMINE COMPOUND EXPLOSIVES