[Pw_forum] ecutrho in phonon calculations

[YY]

Hi Paolo,

Thanks for your reply.  For the phonon calculation, I use a PBE-US PP
(Fe.pbe-sp-van_ak.UPF, and Se.pbe-van.UPF).
According to your suggestion, I increase the ecutwfc, and do several
tests.  Unfortunately, it seems there are still convergence problem:

ecutwfc  ecutrho  f1(cm-1) ... f12(cm-1)
100        1200      106.5    ...  329.5
100        2400      127.2    ...  339.8
200        2400      127.8    ...  341.2
200        4800        32.9    ...  319.8

Could there be any other reasons to cause this convergence problem?

Yuyang at RPI

On Mon, Apr 16, 2012 at 11:52 AM, Paolo Giannozzi
<giannozz at democritos.it> wrote:
> On Thu, 2012-04-12 at 16:07 -0400, YY wrote:
>
>> But I find there is no simple convergence trend while increasing
>> ecutrho.  When I use 35 Ry ecutwfc, and fix all the other parameters,
>> the results are like this:
>>
>>    ecutrho       f1(cm-1)      ....  f12(cm-1)
>>    420            40.475                324.062
>>    600           123.746               350.727
>>    900           -251.876              306.721
>
> unfortunately in at least some [ and I am afraid, in many ] cases,
> a rather high cutoff is needed anyway to get decent phonons. If
> this happens, the advantage of using USPP becomes dubious, given
> their significant computational overhead wrt Norm-Conserving PPs.
> Which USPP did you use ?
>
> P.
>
> --
> Paolo Giannozzi, IOM-Democritos and University of Udine, Italy
>
>