Single‐Reference Coupled‐Cluster Calculations of the Triplet Ground‐State O 2 Dissociation Potential

Three single‐reference coupled‐cluster single and double (CCSD) substitution models, CCSD + T(CCSD), CCSDT‐1, and CCSD(T), exact through the fourth order of perturbation theory, are tested on the triplet ground‐state dissociation potential of O 2 . For the unrestricted Hartree‐Fock (UHF) initial approximation, the interatomic distance region R ˜ 2.4–2.7 a.u. is problematic due to large multireference character, and beyond R ˜ 3.0 a.u. spin contamination causes difficulties. To a large extent these deficiencies are cured by the coupled‐cluster treatments. Fourth‐order triples as well as higher‐order singles and doubles play a significant role. A more flexible basis set than DZP is important, too. None of the methods tried reproduce the experimental potential curve to the desired accuracy (2 kcal/mol). However, in a 6–311 + + G(2df, 2p) basis, the CCSD(T) model does give spectroscopic constants that are correct within 0.2 eV for the dissociation energy D e , 70 cm −1 for the harmonic vibrational frequency v e , and 0.5 cm −1 for the first anharmonicity constant v e X e . This model also reproduces the potential throughout the dissociative region to better than 0.2 eV. Overall CCSD + T(CCSD) is considerably less satisfactory than CCSD(T); CCSDT‐1 is similar to CCSD(T) in a DZP basis, but large basis calculations proved to be too time consuming.