Subject: Re: CCL:Pseudo-Jahn-Teller Effect

From: Anton van Oosten <[email-masked]>
Date: Thu, 17 Mar 94 14:35:51 +0100

Bill Hobey writes:

>In reply to Anton Van Oosten's question, I interpret the Jahn-Teller Effect
>as the coupling of two degenerate electronic states through a vibrational
>mode of motion, while the Pseudo-Jahn-Teller Effect is the coupling of two
>nearly degenerate electronic states through a vibration. Mathematically,
>the only difference is that the diagonal matrix elements (in, say, a
>perturbation treatment) are equal in one case, and differ to a "small"
>extent (of the order of vibrational energies) in the other. Physically,
>there appears to be no significant difference between the two cases.

There is a more important mathematical difference: the JT effect is first order 
in deformation (or vibration) and the Pseudo-JT effect second order.

But I will restate my original point more precisely. 

The JT theorem predicts that the energy of a degenerate state will always be 
lowered, in first order, by a symmetry breaking deformation. In its most general 
formulation, the theorem is derives from symmetry considerations and the 
Hellmann-Feynman theorem. 

In some cases, but not always, symmetry breaking also occurs if there is no 
degeneracy, but now in second order. Is there a Pseudo-Jahn-Teller rule that 
*predicts* whether this will occur or not ?

_____________________________________________________
Anton Van Oosten
University of Linkoeping     Internet: [email-masked]
IFM/FOA, FOA-huset           Phone:    +46  13 282380
S-581 83 LINKOEPING SWEDEN   Fax:      +46  13 142337       
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