The equilibrium structure and infrared spectrum of HNO3 (nitric acid) and its protonated forms have been determined using double-ζ plus polarization and triple-ζ plus double polarization basis sets in conjunction with several ab initio electronic structure methods. Namely, the self-consistent field (SCF), second order Møller-Plesset perturbation theory (MP2), and the single and double excitation coupled-cluster (CCSD) methods have been used. Results using the CCSD(T) method, which includes a perturbational estimate of the effects of connected triple excitations, are also presented. Accurate energy differences have been determined by computing CCSD(T) energies using large atomic natural orbital basis sets. Four different isomers of H2NO3+ have been investigated, and it is found that the lowest energy form of protonated nitric acid corresponds to a complex between H2O and NO2+, consistent with earlier theoretical and experimental studies. There are two isomers of the complex, differing by rotation of H2O relative to NO2+, that are very close energetically (Δ < 0.5 kcal/mol), although only the planar form is found to be a minimum on the potential energy surface. The other two isomers of H2NO3+ that have been investigated both correspond to local minima on the potential energy surface and are very close in energy. These two isomers are about 20 kcal/mol less stable than the complex. The proton affinity of nitric acid is computed to be 182.5 ± 4.0 kcal/mol, which is somewhat larger than the recent experimental estimate of Cacace et al. (168 ± 3 kcal/mol). The binding energy of the complex is determined to be 17.3 ± 2.0 kcal/mol. © 1992 American Chemical Society.