Torsional energy levels of nitric acid in reduced and full dimensionality with ElVibRot and Tnum.

The internal rotation (nu(9)) of nitric acid is studied using several ab initio models including 1D models and full dimensionality (9D) ones. For the 9D calculations, an adiabatic separation between the large amplitude motion, i.e. the torsion, and the eight other modes is performed. The potential energy surface is calculated at CCSD(T) level with a triple zeta atomic basis set and is expanded in a Taylor series up to the second-order along the torsional path. The vibrational calculations are performed with the codes ElVibRot and Tnum. Our main results show that the torsional energy levels and the corresponding tunneling spliting are in good agreement with the experimental ones. Furthermore, the resonance between 2nu(9) and nu(5) (NO(2) in-plane bending) is also well described with full dimensionality models.