We report second-order Moller-Plesset (MP2) and density functional (DFT) calculations of the rotational barrier and torsional potential for intramolecular rotation of the nitro group in the energetic system 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The calculated rotational barrier is 11.0 and 17.9 kcal/mol at the MP2/ 6-311G** and B3LYP/6-311G** levels, respectively, two to three times higher than that of nitrobenzene and nitroethylene. This is attributed to very strong intramolecular hydrogen bonding in this molecule, where the oxygen-hydrogen bond is 1.7-1.8 Angstrom at the equilibrium structure and well below the van der Waals limit of 2.62 Angstrom. The two-dimensional torsional barrier is determined to be 10.5 kcal/mol at the B3LYP/6-311G** level, showing the effects of substantial nuclear relaxation. These results should shed further insight on the unusual stability of TATB.