It was pointed out in the preceding chapter that correlation diagrams can be read from right to left as easily as from left to right. This is certainly true from the formal viewpoint of orbital symmetry, but does not preclude the need to examine the geometric and energetic consequences of the nuclear motions involved. Thus, for example, the necessity for including a conrotatory (a 2)displacement in the reaction coordinate for the cyclization of s-cis- butadiene (Fig. 5.1) implies that a coordinate with the same irrep has to be incorporated into the reaction coordinate for ring opening of cyclobutene. However the energetic requirements of the nuclear motions involved differ greatly. Internal rotation about the central bond of butadiene — with concomitant conrotation of its terminal methylene groups — is quite facile, whereas the reverse of the same motion in cyclobutene is opposed by a substantial restoring force.