Achieving Chemical Accuracy for the First Two Singlet Excited States of Molecular Oxygen

The potential energy curves (PECs) for the three lowest electronic states of the O2 molecule were computed using complete-active space self-consistent field (CASSCF), n-electron valence state second-order perturbation theory (NEVPT2) and multireference configuration interaction with Davidson correction (MRCI+Q) methods with the atomic natural orbital (ANO) type ano-pVXZ (X = D, T, Q, 5) basis set family with three active spaces: CAS(8,6), CAS(10,8), CAS(12,10). A dual-level extrapolation to the complete basis set (CBS) limit scheme was applied, treating the CASSCF reference and the dynamic correlation contribution independently. Adiabatic excitation energies (Te and T0) and spectroscopic parameters were obtained from the extrapolated curves. At the MRCI+Q CBS level, the Te values deviated from experiment by −30.10 cm−1 for the first excited state (aΔg 1) with CAS(8,6) and −23.26 cm−1 for the second excited state (bΣg+ 1) with CAS(12,10), while the corresponding T0 deviations were −41.70 cm−1 and −35.49 cm−1, respectively. A non-monotonic dependence of the excitation energies on the active space size was identified and attributed to the asymmetric orbital composition of the intermediate CAS(10,8) space. The present results demonstrate that the ano-pVXZ basis set family combined with a systematic active space investigation can achieve an accuracy comparable to calculations employing additional core-valence and scalar relativistic corrections.