The 'tailored' CCSD(T) description of the automerization of cyclobutadiene

► The application of the CCSD(T) approach has been extended to multireference cases by 'tailoring' the underlying CCSD approach. ► The resulting TCCSD(T) method is believed to provide electron correlation energies of 'genuine' multireference quality while having CCSD(T) computational demands. ► The method derived is used for calculating the entire potential energy surface for the automerization of cyclobutadiene. The calculated reaction barrier height lies in the middle of the experimental range.