Gozem et al. have reported photoisomerization reaction pathways for PSB3 at the CASSCF and CASPT2 levels of theory. These pathways connect the cis and trans isomers of PSB3 with a CX seam between the S 0 and S 1 states. The most important geometric parameters that characterize these pathways are the bond-length alternation (BLA) coordinate, the C 1 -C 2 single-bond torsion, and the C 2 =C 3 double-bond torsion. [See Fig. for the PSB3 structure.] The reaction pathway computed at the CASSCF level shows a decrease in the BLA as the molecule moves away from Frank-Condon region on the S 1 potential surface. Following that, the central double bond (C 2 =C 3 ) starts to twist and the BLA coordinate increases again until the CX region is reached, at which point a nonadiabatic transition returns the system to the ground state. For the CASPT2 reaction path, however, the same BLA change in the Frank-Condon region leads to a shallow local minimum (called "MIN trans " in what follows), which corresponds to a structure with a rotated C 1 -C 2 single bond. According to the CASPT2 calculations, the C 2 =C 3 torsion is activated by overcoming a small barrier.