[Pw_forum] negative (alpha)^2*F(w)

[Stefano Baroni]

As discussed, many times in this forum, as well as in many textbooks,  
negative squared ferquencies are a signal of a mechanical instability.  
Are you sure that you want to spend a lot of computer resources  
calculating the supercondictive properties of an instable (and, hence,  
non-existent unless very special conditions occur) system? SB

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swift text written and sent on the go

On 13/ago/2009, at 11.00, Ali Tavana <anavat_ila at yahoo.com> wrote:

> Dear QE users,
>
> I have a question about e-ph lambda, calculated by "matdyn" program.  
> I've found that for some frequencies the quantity "(alpha)^2*F(w)/w"  
> or "dos_a2F(w)/w" can be negative, most probably because of the  
> negative (imaginary) frequencies (I can't believe a2F(w) become  
> negative!). In "matdyn" program these negative values are considered  
> in lambda calculations. Should one remove these negative values as  
> these imaginary frequency modes can never exist in the system? then  
> where are those phonon branches? I think negative values of  
> frequency can show in some manner more strong electron structure  
> interaction, so what do you suggest one should do?
>
> regards,
> Ali
>
>
>
>
>
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