Polyatomic system high-symmetry configuration

The necessary and sufficient condition of instability (lack of minimum of the AP) of high-symmetry configurations of any polyatomic system is the presence of two or more electronic states, degenerate (except 2-fold spin degeneracy) or pseudodegenerate, which interact sufficiently strong under the nuclear displacements in the direction of instability . 

Somewhat paradoxically, symmetry is seen to play an important role in the understanding of the Jahn-Teller effect, the very nature of which is symmetry destruction [52], In a recent review the original paper published by Jahn and Teller [53] was called one of the most seminal papers in chemical physics [54], Only a brief discussion of this effect will be given here for more detail we refer the reader to References [55-59], Bersuker says that all structural instabilities and distortions of high-symmetry configurations of polyatomic systems are of Jahn-Teller origin (here he also refers to other related effects, such as the Renner-Teller effect and the pseudo-Jahn-Teller effect—they will be mentioned later). Bersuker likes to call this the... 

Thus, with these proofs two important additions to the previous traditional understanding of the JTE emerged (1) Any polyatomic system may be subject to the JTE, and (2) if there are instabilities and distortions of high-symmetry configurations, they are due to, and only to the JTE. Together with the previously achieved understanding of the role of quadratic terms of the vibronic coupling, the extended formulation of the JTE that includes the latest achievements in this field is as follows [1,8] ... 

The generalized Jahn-Teller effect (JTE) [including proper JTE, pseudo-JTE (PJTE), and Renner-Teller effects (RTE)] in its present understanding is a local feature of any polyatomic system that describes its properties in high-symmetry configurations... 

The JT theorem [40] states that if in the high-symmetry configuration of a nonlinear polyatomic system the electronic state F is degenerate, there is at least one low-symmetry coordinate in the linear terms of the vibronic coupling that... 

In a series of papers (see Refs. 6, 7, 34,42, and 63), beginning in 1980, it was proved that for any polyatomic system in the high-symmetry configuration... 

If > 0 for the high-symmetry configuration of any polyatomic system, then a negative curvature, K=(K +K)<0, can be achieved only due to the negative vibronic coupling component K, and only if It follows that any SSB of... 

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