An all-electron CASPT2 study of Ce(eta(8)-C8H6)(2)

CASSCF wave functions and CASPT2 energies are presented for the ground and 15 and 31 excited states of the D2d and D2h conformers respectively of the f-element sandwich molecule CePn2 (Pn = η8–C8H6). The ground states of both conformers are found to be strongly multiconfigurational, analogous to cerocene, CeCOT2 (COT = η8–C8H8), as reported in Journal of Physical Chemistry A113 (2009) 2896. Investigation of the nature of the multiconfigurational character shows that the analogy with cerocene is indeed strong. It is shown that the occupations of the natural orbitals produce a more reliable description of the CePn2 ground state than does the configurational admixture, and that these occupations can be used to deduce a value for the total f-density in the system. This density is found to be slightly lower than that of cerocene, and is in good agreement with that found experimentally. A density functional theory calculation of the same system is able to broadly capture the important features of the electronic structure, but is unable to replicate the experimental value of the total f-density. CASPT2 calculations reproduce the energy of the first band in the UV/Vis spectrum of CePn*2 (Pn* = η8–C8Me6) to within 0.01 eV. Although this extremely good agreement is probably somewhat fortuitous, it highlights the fact that multiconfigurational methods are required in order to model systems such as CePn2 to a high level of accuracy.