Force field of para- and metabenzyne diradicals: A multireference coupled-cluster study

The reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that exploits a modest size MR CISD wave function as an external source for a small subset of the most important (primary) triples and quadruples to account for quasidegeneracy due to a MR nature of the states considered, as well as its RMR CCSD(T) version corrected for the secondary triples, are employed to compute fundamental vibrational frequencies for the para- and metabenzene diradicals. A comparison is made with the available experimental data and with other methods, namely, the state selective or Mukherjee CCSD and completely renormalized, size-extensive version of CCSD(T), the so-called CR-CC(2,3), methods. Both the restricted Hartree-Fock and multiconfiguration self-consistent-field (MCSCF) molecular orbitals (MOs) are employed. The four-reference RMR CCSD using MCSCF MOs gives particularly satisfactory results. The breakdown of CCSD(T) and shortcomings of some other approaches are pointed out and the role of invariance with respect to MO rotation is analyzed. Correlation with triplet-singlet splittings is also pointed out.