All quantum-chemical calculations were performed using Dunning's aug-cc-pVTZ basis set. The DFT calculations have been performed with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. In all quantum-chemical calculations, Orca's 'very tight' self-consistent field convergence criteria were chosen. The CCSD(T) and OO-CCD calculations otherwise used Orca's default settings. In particular, the oxygen 1s core orbitals are frozen in the coupled-cluster treatment.