Abstract Three local minima on the potential energy surface of the nitric acid dimer are located by computations using hybrid density functional theory and second-order Moller–Plesset perturbation theory with a number of basis sets. The global minimum is a cyclic C 2 h structure, with a binding energy of 8.4 kcal/mol, in which the monomers are bound by two strong, nearly linear O–H⋯O hydrogen bonds. Both the other structures are non-planar; one has two hydrogen bonds and the other only one. The best calculations predict that the two higher-energy structures are separated from the most stable one by at least 3.4 kcal/mol, and from each other by not more than 0.4 kcal/mol.