Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for neutral HOONO and its cation employing quadratic configuration interaction methods and correlation-consistent basis sets. The lowest energy structure of HOONO+ found is a planar conformer on the (2)A' potential-energy surface with a cis-trans arrangement [tau(OONO)=0degrees and tau(HOON)=180degrees]. This is quite the opposite from neutral HOONO, which prefers a cis-cis arrangement because of hydrogen bonding. The adiabatic ionization potential for HOONO is predicted to be 260.0+/-1 kcal mol(-1) (11.3 eV) at the complete basis set limit. A planar ion-molecule complex formed from HO2 and NO+ was located on the (2)A" surface. The binding energy of this complex relative to HO2 and NO+ is estimated to be 18.1+/-1 kcal mol(-1) at the complete basis set limit. Excited states of the HOONO+ ion were also calculated with quasi-restricted Hartree-Fock and unrestricted Hartree-Fock coupled-cluster methods including connected triple excitations. (C) 2003 American Institute of Physics.