Hula-twist (HT) motion of tZt-penta-2,4-dieniminium (PDI) on the S-1 surface is studied on the basis of complete active space self-consistent field (CASSCF) theory. An S-1/S-0 crossing region (a segment of the S-1/S-0 conical intersection hyperline) along a HT coordinate (simultaneous rotation of the central double bond and an adjacent single bond) is characterized. One-dimensional relaxed scan calculation along the postulated HT coordinate shows no barrier up to the S-1/S-0 crossing region, suggesting the possible involvement of the HT process in a fast photochemical reaction in constrained states. (c) 2006 Elsevier B.V. All rights reserved.