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

► The barrier height for automerization of cyclobutadiene is investigated by means of BS and MR approaches. ► UHF Natural Orbital (UNO) based Mukherjee's state-spefic MRCC (MkCC), named UNO-MkCC calculations perform well. ► Most UDFT methods even long-range corrected ones provide disappointing results for the present reaction.