[Pw_forum] Exceptionally large charge density cut-off

[Subhradip Ghosh]

Thanks Paolo. Does that mean one should not use USPP for phonon calcs at
all? I guess it varies from case to case. I have seen people using USPP
for phonon calculations though. For example, Dal Carso and Gironcoli, PRB
62, 273 (2000) has calculated phonons in Fe and Ni with USPP. Bungaro,
Rabe and Dal Carso used USPP for Ni and Mn in Ni2MnGa phonons (PRB 68,
134104 (2003)).

This is bit strange as to why USPP would fail for phonons. In that case,
the applicability of USPP would be really limited.



> Subhradip Ghosh wrote:
>
>> [..] I find that phonon frequencies at Gamma and [1/2 1/2 1/2] points
>  > do not converge even after a large charge density cut-off although with
>> respect to k-mesh size and kinetic energy cut-off,
>> the convergence was achieved with reaonable values of these parameters.
>> [...] The lowest lying acoustic mode is not yet converged to within 1%
>> while the other modes have converged.
>>
>> This is very surprising because when I looked into the archive and the
>> relevent literature I didn't find any reference to such a behavior.
>
> you are using ultrasoft pseudopotentials, right? the calculation of
> phonon frequencies - especially acoustic phonons - with USPP is quite
> tricky. Large cutoffs for the charge density are needed, sometimes
> so large to make the calculation impractical. Unfortunately I am not
> aware of any remedy. Last time I tried to use USPP in a phonon
> calculation, I gave up. I am trying again right now...
>
> Acoustic modes are sensitive to small violations of the Acoustic Sum
> Rule (i.e. of translational invariance), and this is well known (even
> in the case of norm-conserving PP, where the violation of the ASR comes
> from the discretization on a grid of the exchange correlation
> potential). At Gamma this is typically fixed by imposing the ASR on
> the dynamical matrix, but this may not be sufficient.
>
> Paolo
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy
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>


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              Dr. Subhradip Ghosh
              Assistant Professor
              Department of Physics
            Indian Institute of Technology
               Guwahati,Assam-781039
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