LAUSR

We present a large set of vertical excitation calculations for the ortho‐nitrobenzaldehyde (oNBA) molecule, which exhibits a very challenging excited‐state electronic structure like other nitroaromatic compounds. The single‐reference methods produce mostly consistent results up to about 5.5 eV. By contrast, the CAS second‐order perturbation theory (CASPT2) results depend sensitively on the employed parameters. At the CAS self‐consistent field level, the energies of the bright ππ* states are strongly overestimated while doubly excited states appear too low and mix with these ππ* states. This mixing hampers the CASPT2 step, leading to inconsistent results. Only by increasing the number of states in the state‐averaging step to about 40—to cover all bright ππ* states embedded in the double excitations—and employing extended multistate CASPT2 could CASPT2 results consistent with experiment be obtained. We assign the four bands in the molecule's spectrum: The weakest band at 3.7 eV arises from the nNO2π* states, the second one at 4.4 eV from the ππ* ( Lb ) state, the shoulder at 5.2 eV from the ππ* ( La ) state, and the maximum at 5.7 eV from the ππ* ( Ba/Bb ) states. We also highlight the importance of modern wave function analysis techniques in elucidating the absorption spectrum of challenging molecules.