A broken-symmetry study on the automerization of cyclobutadiene. Comparison with UNO- and DNO-MRCC methods

The barrier height for automerization of cyclobutadiene (CBD) is examined by broken-symmetry (BS) HF, post-HF, and density functional methods. To remove the spin contamination error, we use an approximate spin projection (AP) method. Comparison between the results of BS methods and those of the localized UHF natural orbital (ULO) based Mukherjee’s state-specific MRCC (ULO-MkCC) method shows that spin-unrestricted Brueckner doubles including perturbative triple excitations (UBD(T)) with the AP correction performs well within a framework of single reference approaches. On the other hand, UDFT methods except for UBLYP and UM06-2X give negative barrier heights, which are not consistent with the both the experimental and MRCC results.