[QE-users] Getting rid of negative values of phonon frequencies for lambda.x calculation

[mkondrin at hppi.troitsk.ru]

Dear developers and users!

I could somehow resolve this issue. The solution was using q2r.x and 
matdyn.x with option la2F=.true. It is briefly mentioned in the 
enlightening tutorial of S. Ponce. This routine (alphough quite lengthy - 
it takes 15 minutes for cell containing 4 atoms and 10 q-points) produces 
"lambda" file with logarithmic frequency and lambda constant. Using 
McMillan formula with this input the Tc can be evaluated. It turns out that 
it more or less corresponds to the values produced by lambda.x program. The 
benefits of this routine is that it is "bullet proof" against negative 
frequencies.

Still I have a question. If it will come to publication, how can I cite 
matdyn.x program and algorithm it used to calculate Tc (with this 
accounting for negative frequencies)?

Sincerely yours,
M.V. Kondrin 

On Sep 13 2022, mkondrin wrote:

>Dear QE developers and users!
>
>I encountered a problem calculating Tc of certain material. During 
>calculations of its phonon dispaersion I obtain negative (bat small in 
>absolute value) of three acoustic modes in Gamma point (all other modes 
>are positive). So, this prevents obtaining Tc values of this material 
>due to the diverging logarithm of negative values. How to circumvent 
>this issue? I simply delete negative sign of this modes in elph file. Is 
>it OK?
>
>Thank you in advance.
>
>Sincerely yours,
>M. V. Kondrin
>
>
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