Welcome to the newly created homepage of the Jahn-Teller (JT) effect and related activities! This is hoped to give an overview over this rich and still expanding field and deepen your interest in relevant details. A rather systematic introduction to theory and experiment for this effect and the related pseudo Jahn-Teller (PJT) effect should be provided by the wikipedia pages, available through the navigation bar at the left. Further links in this bar point at a list of JT-related conferences and organizational details as well as to the most recent of such conferences which were held virtually in May 2023 at York University in Toronto (Canada) and May 2025 simultaneously at York University in Toronto (Canada) and University of Louisville in Kentucky (USA) .

The two logos in the banner at the top of the page highlight a prototypical JT situation, the so-called Exe Jahn-Teller effect. By the very nature of the Jahn-Teller theorem the two-fold electronic (spatial) degeneracy is lifted in first order of the displacements, making up for a double-cone structure of the potential energy surfaces, which is visible in the center of the left drawing (conical intersection).  Motion around the conical point in a closed loop is known as pseudorotation which is to be distinguished from a real (rigid-body) rotation because the atoms undergo (normally small-amplitude) near-circular motion around their reference points. This is the degeneracy point mentioned earlier, and the motion is illustrated in the right drawing for the simple case of a triatomic system, where the pseudorotational motion proceeds around the corners of an equilateral triangle. An illustration for a larger system is provided here.

If you have questions or comments, send us an email:

• Horst Köppel (fu127@uni-heidelberg.de)

• Pablo García-Fernández (garciapa@unican.es)