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

[Nicola Marzari]

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
> 
> 
> _______________________________________________
> users mailing list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
> 


-- 
----------------------------------------------------------------------
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