The ABINIT Users Mailing List ( CLOSED )

[amall ramanathan <paddu46@yahoo.com>]

Dear abinit users,

I have been computing phonon dispersions in bcc metals and have been sucessful in getting the phonon freqencies.I would like to use the program band eps to draw the phonon dipersion curves for Gamma H Gamma N .I tried to modify the trf2_6.in file which is in the tutorial and which applies to Fcc crystals, but was not successful.Maybe I got the scale factors wrong.Does anybody have an input file equivalent of ' trf2_6.in' for bcc metals that they could share?

Thanks and good luck to all.

       with regards,

          Amall Ramanathan.

delaire@caltech.edu wrote:

> Dear Abinit users,
>
> I have been computing phonons in bcc vanadium, but I am running into some difficulties and I could use some
>  advice. I am using Abinit (sequential) 4.6.5.
>
> First, I computed phonon dispersions for Nb using the input files in /Test_phys/t11.in and with the FHI GGA-PBE pseudopotential, and these came out in good agreement with the literature. 
> I then adapted these input files for the case of bcc Vanadium. I am also using the FHI GGA-PBE in this case. I performed a volume optimization and I am using the optimized cell parameter (5.8074 Bohr) in the phonon calculation. Here are the issues I am running into:
>
> 1) when using ecut=17Ha for V (same value used for Nb) and all other parameters identical as for Nb (same qpt list, same kpt grid, same tsmear-0.015Ha), I get an error (something like) "SCF convergence error, failed to converge after 13 restarts..." during one of the response function calculations.
>
> 2) I tried to change the iscf parameter from its default value to iscf=3 (that was the setting to compute fcc Al phonons in workshop tutorial), but this makes the
>  SCF cycle diverge (goes through all nsteps, but delta_E is something like 10^65...!)
>
> 3) when increasing the value of ecut from 17Ha to 26Ha (which seems more appropriate for V based on GS energy convergence study), with the default iscf=5 (and all other parameters identical to t11.in), the response-function calculations converge ok, if I raise nstep to 50. But the phonon dispersions display some anomalous behavior with transverse acoustic branches showing begative slopes at Gamma, leading to negative frequencies from Gamma to H and Gamma to N.
>
> 4) I have tried to increase the kpt mesh from 8x8x8 to 16x16x16 and (independently) to decrease the value of tsmear from 15mHa to 5mHa, but both lead to a failure of the convergence (using default iscf=5) as in 1)
>
> 5) when going to iscf=3 in case 4), the SCF blows up as in 2)...
>
> Here are my questions:
> -has anyone managed to calculate good phonon dispersions/DOS for bcc vanadium?
> -what parameters are
>  likely to improve on the negative slopes/frequencies I am getting (case 3 above)?
>
> Sorry for posting such a long email, and thanks in advance for any help.
> Regards,
> Olivier Delaire.

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