Adiabat‐to‐Diabat Angle in Seam Space: Renner‐Teller‐Type and Pseudo‐Jahn‐Teller‐Type Problems

In this study, we examine the adiabat‐to‐diabat (ATD) angles for trajectories in 2‐dimensional vibrational subspace of the seam space of two degenerate states. In circulating around the tangential touching degeneracy center, the ATD angle is changed by or 0, similar to the Renner‐Teller problem and the pseudo‐Jahn‐Teller problem, respectively. These ATD angle profiles may be indistinguishable from those of circulating multiple conical intersections or a pseudo‐Jahn‐Teller center. Methods to discern those seemingly indistinguishable cases are proposed. A sharp zigzag variation of the ATD angle is seen as a feature for trajectories that graze a pseudo‐Jahn‐Teller‐type tangential touching center, in contrast to the monotonic steep variation for grazing a conical intersection or a Renner‐Teller‐type tangential touching center.