Electron affinity of the O₂ molecule: CCSD(T) calculations using the optimized virtual orbitals space approach

The electron affinity (EA) of the oxygen molecule is calculated by the CCSD(T) method using the optimized virtual orbitals space (OVOS) technique by which the dimension of the original space of virtual orbitals can be significantly reduced. Extended basis sets, up to the doubly augmented correlation consistent d‐aug‐cc‐pV6Z basis sets for O 2 and O at their experimental geometries are used. We demonstrate that even when the space of virtual orbitals is reduced to 50% of the full space, the resulting EA of the O 2 molecule is accurate to within 0.01 eV. At the same time, the computational effort is reduced by about an order of magnitude. With OVOS reduced to 60% of the full virtual space, results are almost accurate. Considering the complete basis set limit with so reduced OVOS, corrections for the core correlation, vibrational and relativistic effects, the electron affinity of O 2 is 0.452 ± 0.01 eV. This value agrees very well with the full virtual orbital space, EA = 0.446 eV, and with the recent experimental value, EA = 0.448 ± 0.006 eV.