Ciao Aldo,
a meaningful/useful test would be this - take a simple case of a CN
system - it can be as simple as an infinite 2-atom primitive cell of a
graphene-like sheet (or a 2-atom primitive FCC cell (like GaAs)).
Doesn't matter if it's not that physical. You also do not need fine
k-point sampling. Shake one of the 2 atoms quite a bit - 10-15% of their
typical bond lengths.
For this system, plot the total force on 1 atom, and the trace of the
stress tensor, as a function of cutoff on the wavefunctions (30, 40, 50,
60, 70, 80, 100 Ry) for a couple of duals (i.e. ecutrho six or 8 times
larges than ecutwfc). If you feel good about doing it, look also at the
frequencies of a phonon - maybe not Gamma, but at a zone boundary.
This will tell you what cutoffs you need for a structural relaxation.
Let me also repeat that the convergence on the total energy is not a
necessary condition - you can have wonderful and accurate pseudos whose
total energy is still changing at a cutoff that is good for forces and
stresses.
nicola
On 03/04/2020 10:43, Aldo Ugolotti wrote:
Dear Nicola,
I share with you the perplexity about the unusual values for the
cutoffs, that's why I wished to understand the origin of it and whether
it could be improved or fixed (other than just setting such cutoff and
keep going). About checking the pseudo versus the total energy, it might
be not so relevant but indeed it is a necessary condition.
In my tests I just run scf calculations with different cutoffs/k-mesh
with fixed geometry and extract the total energy (i.e. fixing the
precision rather than the accuracy, in the language of Science, 351,
2016, paper). I consider the minimal cutoff the one at which the total
energy is within ~ 1mRy from the value of the calculation with the
highest cutoff. I can also extract the corresponding total force or stress.
Finally, I definitely agree with you about the "hardness" of
pseudo-generation; the results I was reporting are from the checks on
the SSSP pseudos (or pslibrary v 0.1<->1.0.0 ones, in general, which
yield similar numbers), the only difference lies in taking into account
a system more complex that a single atom. In my particular case, I did
consider a small patch of graphene-like C,N compound.
Hence I wondered if there was any modification, given the possible
differences between the isolated atom and the small patch, to improve
the effectiveness of the pseudo (hence, not just hardness, but a proper
size of the dual). My first guess was to check the transferability of
the pseudo, which maybe was not enough to cover the electronic
configuration of the patch. That's what I could not test with ld1.x
Bests,
--
----------------------------------------------------------------------
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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