Ground state calculations of di-π-cyclooctatetraene cerium

Quantum chemical calculations are presented which predict that in the ground state of di-π-cyclooctatetraene cerium (cerocene) the Ce ion is almost entirely in a 4f1 configuration corresponding to Ce3⊕(C8H1.5■8)2. The 4f electron forms with an electron of the ligand e2u highest-occupied molecular orbital a 4f1e32u singlet in close analogy to a Kondo ion in a metal. Due to coupling of the 4f1e32u with the 4f0e42u configuration, the latter corresponding to Ce4⊕(C8H2≤8)2, the splitting between the ground state singlet and the first excited triplet is of the order 0.5 eV. The self-consistent-field and multiconfiguration self-consistent-field parts of the calculations are done by employing recently developed pseudopotentials for cerium using basis sets of up to 626 basis functions. The correlation energy is accounted for by means of various correlation-energy density functionals and also by limited coupled electron-pair approximation calculations. Similar results are found in both cases.