[Pw_forum] "Raman activities" do not converge

[xirainbow]

Dear Lorenzo Paulatto:
              Thank you very much for your suggestion:)

> Compare the output of pw.x at different cutoffs to be sure, my bet is
> that at that specific value of ecutwfc some symmetry operations are
> incompatibles with the FFT grid.
             The "FFT vs Symmetry" condition really happens.
              However, it has no relation with "Ramman activity". See
the following table.

Using "C.pw-mt_fhi.UPF" :
ecutwfc(Ry)      Freq(cm-1)  Raman activity      Sym. Ops.
 65                 1330.65          68.9754               24
100                1330.23          68.9674               24
110                1330.50      3066.0811               48
120                1330.48          15.1570               48
130                1330.47          68.9545               48
140                1330.48          68.9536               48
150                1330.48          68.9541               48

> In general, I found that when you have problems converging response
> (phonon, epsilon, raman...) calculations it's better to use a much
> stricter conv_thr (i.e. 1.d-12).
> Sometimes I even reduce the hard-coded
> threshold (ethr) in PHonon/PH/set_defaults_pw.f90
              I follow your suggestion and do the calculation.   When
I finish it , I will put it here:)
              There is a parameter tr2_ph in phonon.input.   Could
you please tell me the relation between "ethr" and  "tr2_ph"?

Thank you again:)

-- 
____________________________________
Hui Wang
School of physics, Fudan University, Shanghai, China




On Fri, May 31, 2013 at 12:06 AM, Lorenzo Paulatto
<lorenzo.paulatto at impmc.upmc.fr> wrote:
> On 05/30/2013 05:29 PM, xirainbow wrote:
>>             I  changes the pseudopotential from "C.pw-mt_fhi.UPF" to
>> "C.pz-hgh.UPF" and get the expected "Raman activity" for different
>> ecutwfc. However, the results of "ecutwfc=110" are still higher than
>> others.
>>
> Compare the output of pw.x at different cutoffs to be sure, my bet is
> that at that specific value of ecutwfc some symmetry operations are
> incompatibles with the FFT grid. It can make convergence harder. In
> general, I found that when you have problems converging response
> (phonon, epsilon, raman...) calculations it's better to use a much
> stricter conv_thr (i.e. 1.d-12). Sometimes I even reduce the hard-coded
> threshold (ethr) in PHonon/PH/set_defaults_pw.f90
>
> bests
>
>
> --
> Dr. Lorenzo Paulatto
> IdR @ IMPMC -- CNRS & Université Paris 6
> phone: +33 (0)1 44275 084 / skype: paulatz
> www:   http://www-int.impmc.upmc.fr/~paulatto/
> mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
>
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