[QE-users] Convergence and the "dual": norm-conserving vs PAW

[Christoph Wolf]

Dear Nicola,

thanks a lot, a very interesting read!

Best,
Chris

On Fri, Sep 7, 2018 at 2:58 PM, Nicola Marzari <nicola.marzari at epfl.ch>
wrote:

>
>
>
> Hi Christoph,
>
>
> a dual of 2 is cutting a lot of corners for norm-conserving
> pseudopotentials (where a dual of 4 is mathematically perfect,
> and one of ~3 could actually work).
>
> Convergence in the total energy for norm-conserving at dual of 4
> is strictly variational, but also not very interesting as a criterion.
>
> The phonon frequencies are an interesting criterion, but are not
> variational.
>
> So, there is nothing fool proof - but a preponderance of evidence
> helps feeling better - this is discussed at length here:
> https://arxiv.org/abs/1806.05609
>
>
>                                 nicola
>
>
>
>
> On 07/09/2018 10:43, Christoph Wolf wrote:
>
>> Dear all,
>>
>> I am afraid that this is a very basic question but I will ask it anyway
>> in the hopes of some pointers. I have recently studied the convergence
>> behavior of a set (Mg and O) pseudos with respect to the phonon frequencies
>> and I encounter a behavior that I quite not understand.
>>
>> I study the convergence of total energy and the highest phonon frequency
>> at q=(0.5,0.5,0.5). I vary the dual=ecutrho/ecutwfc=2,4,8.
>>
>> for the norm-conserving pseudos (standard dual is 4 here) energy
>> converges (monotonously) at a ecutwfc=50 irrespective of the dual. however
>> the phonon frequency only converges (strongly non-monotonously, i.e. in a
>> zig-zag pattern) if (i) ecutwfc=100 and dual 8 OR (ii)ecutwfc=200 and dual 4
>>
>> for the PSLibrary 1.0.0 pseudos the total energy converges (monotonously)
>> at ecutwfc=50 for all duals but interestingly so does the phonon frequency
>> (in excellent agreement with the converged norm.-cons.!). Varying the dual
>> from 2 to 8 leaves the phonon frequencies virtually unchanged. the
>> suggested hardest cutoff for the pseudos (from the file) is Mg: 97/398 -
>> higher than what I found.
>>
>> Now I have read that for phonons and US/PAW often a dual of 8-12 is
>> advised (I think the example is for metals not an insulator as MgO) but I
>> was curious if there is any "fool proof" method to ensure the convergence
>> whilst not risking falling in a "local minima" of a phonon frequency, for
>> example?
>>
>> Thanks for reading all the way down to hear, your help is greatly
>> appreciated!
>>
>> Best,
>> Chris
>>
>> --
>> Postdoctoral Researcher
>> Center for Quantum Nanoscience, Institute for Basic Science
>> Ewha Womans University, Seoul, South Korea
>>
>>
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>>
>
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
>



-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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