[QE-developers] problem in running lambda.x for the calculation of Tc

[Paolo Giannozzi]

On 26/06/2022 18:17, Soumendra Kumar Das wrote:

> I am using quantum espresso for superconductivity Tc calculation. I put 
> this question last week in the QE users forum  but I did not get any 
> response.

your message was held because too large

> I was trying to calculate the superconducting transition temperature of 
> Pb and Al given as trial example in the phonon calculation of QE. all 
> the steps are running successfully. only the last step is giving "NaN" 
> (not a number).
> 
> I am requesting to kindly suggest to me what I should do and why this 
> error is coming.

the utility 'lambda.x' is basically unmaintained, because there are much 
better ways to compute electron-phonon quantities (e.g. using EPW). I 
plan to have a look at the entire electron-phonon stuff sooner or later 
but it is not high in my list of priorities. In the meantime: most 
likely, the NaN comes from a acoustic phonons with "negative" 
frequencies at q=0. These contribute nothing to the lambda, so you may 
just try to set them explicitly to zero, or to a small positive value. 
The lambda.x code is very simple and one can just look inside it to see 
what happens and why

Paolo

> I am sending  both the input and output data for lambda.x calculation. 
> The tutorial file which I was following is also attached here.
> 
> ###########################################################################
> 10  0.12  1    ! emax (something more than highest phonon mode in THz), 
> degauss, smearing method
>      8          ! Number of q-points for which EPC is calculated,
>      0.0000000  0.0000000  0.0000000   1.00  ! the first q-point, use 
> kpoints.x program to calculate
>      -0.2500000 -0.2500000  0.2500000   8.00  ! q-points and their weight
>      -0.5000000 -0.5000000  0.5000000   4.00  !
>       0.0000000  0.0000000  0.5000000   6.00  ! 4th q-point, qx,qy,qz
>      -0.2500000 -0.2500000  0.7500000  24.00  !
>      -0.5000000 -0.5000000  1.0000000  12.00  !
>       0.0000000  0.0000000  1.0000000   3.00  !
>      -0.5000000  0.0000000  1.0000000   6.00  ! the last q-point
> elph_dir/elph.inp_lambda.1 ! elph output file names,
> elph_dir/elph.inp_lambda.2 ! in the same order as the q-points before
> elph_dir/elph.inp_lambda.3
> elph_dir/elph.inp_lambda.4
> elph_dir/elph.inp_lambda.5
> elph_dir/elph.inp_lambda.6
> elph_dir/elph.inp_lambda.7
> elph_dir/elph.inp_lambda.8
> 0.10                     ! \mu the Coloumb coefficient in the modified
>                           ! Allen-Dynes formula for T_c (via \omega_log)
> 
> ######################################################################
> 
>       lambda = 0.242639 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   1.294938 at degauss= 0.005
>       lambda = 0.350909 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   1.845335 at degauss= 0.010
>       lambda = 0.348256 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.071123 at degauss= 0.015
>       lambda = 0.324666 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.196774 at degauss= 0.020
>       lambda = 0.302942 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.275654 at degauss= 0.025
>       lambda = 0.289683 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.340134 at degauss= 0.030
>       lambda = 0.283603 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.397743 at degauss= 0.035
>       lambda = 0.281253 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.449225 at degauss= 0.040
>       lambda = 0.280253 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.493764 at degauss= 0.045
>       lambda = 0.279197 (        NaN )  <log w>=      NaN K  N(Ef)= 
>   2.530433 at degauss= 0.050
> lambda        omega_log          T_c
>     0.24264           NaN                NaN
>     0.35091           NaN                NaN
>     0.34826           NaN                NaN
>     0.32467           NaN                NaN
>     0.30294           NaN                NaN
>     0.28968           NaN                NaN
>     0.28360           NaN                NaN
>     0.28125           NaN                NaN
>     0.28025           NaN                NaN
>     0.27920           NaN                NaN
> 
> 
> -- 
> *---------------------------------------*
> *Thanks and Regards*
> *Soumendra Kumar Das*
> *Postdoc. Research Scholar*
> *School of Physical Sciences*
> *National Institute of Science Education and Research (NISER), 
> **Bhubaneswar*
> *PO- Bhimpur-Padanpur, Via- Jatni, District:- Khurda*
> 
> *PIN – 752050, Odisha, INDIA*
> 
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222