or antisymmetric (green) with respect to the plane. Each doubly occupied reactant orbital can be correlated with a doubly occupied product orbital, which makes the disrotatory reaction pathway energetically favorable and, thus, thermally "allowed." In an electronic state picture, the ground state (ππ) Slater determinants of reactant and product correlate. The reaction only changes the energetic ordering of the highest two occupied molecular orbitals (HOMO and HOMO-1).