At the same level, also CASPT2 calculations were performed using a CASSCF wave function based on an active space containing the 2s and 2p orbitals of both O atoms. The initial guess for the active space was obtained using the atomic valance active space (AVAS) procedure based on R(O)HF orbitals and both CASSCF and CASPT2 employed density fitting. Density functional theory: Most density functional calculations were calculated with the psi4 program (release 1.6.1, the composite results were obtained with version 1.9).