The ground state Ψ12Ψ22 of 1,3-butadiene correlates with the ground state σ2π2 of cyclobutene as demonstrated in the MO correlation diagram above. Ψ1 correlates with π and Ψ2 correlates with σ. Thus the orbitals making up Ψ12Ψ22 must transform into the orbitals making up σ2π2 under a conrotatory mechanism. However, the state ES-1 does not correlate with the state ES-1' as the molecular orbitals do not transform into each other under the symmetry-requirement seen in the molecular orbital correlation diagram. Instead as Ψ1 correlates with π, Ψ2 correlates with σ, and Ψ3 correlates with σ*, the state Ψ12Ψ2Ψ3 attempts to transform into π2σσ*, which is a different excited state. So ES-1 attempts to correlate with ES-2'=σπ2σ*, which is higher in energy than Es-1'. Similarly ES-1'=σ2ππ* attempts to correlate with ES-2=Ψ1Ψ22Ψ4. These correlations can not actually take place due to the quantum-mechanical rule known as the avoided crossing rule. This says that energetic configurations of the same symmetry can not cross on an energy level correlation diagram. In short, this is caused by mixing of states of the same symmetry when brought close enough in energy. So instead a high energetic barrier is formed between a forced transformation of ES-1 into ES-1'. In the diagram below the symmetry-preferred correlations are shown in dashed lines and the bold curved lines indicate the actual correlation with the high energetic barrier.