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
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