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,
--
Aldo Ugolotti, Ph.D.
Post-doc fellow
Materials Science Dept. U5,
Università degli Studi di Milano-Bicocca
via Cozzi 55,
20125 Milano (MI)
Italy
e-mail: [email protected]
On 02/04/20 19:00, Nicola Marzari wrote:
On 02/04/2020 18:16, Aldo Ugolotti wrote:
Dear Nicola,
thanks for your reply. I mentioned the energy criteria for
convergence as the simplest one to verify.
Dear Aldo,
indeed, it is simple, but also it is not relevant, so there aren't
many conclusions that can be drawn from it.
Regarding the pseudos you mentioned (PBE), those available at the
materialscloud portal, I have checked them either in the atomic case
and for a simple system of interest. In the latter case, I found that
the required minimal cutoffs are ~90/225 Ry while in the former case
the results of ld1.x (for Hamiltonian evaluation with Bessel
functions) are consistent with the numbers you reported.
For the SSSP, the cutoffs are based on convergence of phonon
frequencies, stress, and formation energies; the systems considered
are the elemental solids of Cottonier (we have protocols also for more
complex systems). Of course, you might need tighter cutoffs for your
goals, or you could afford looser ones. Not sure what your tests are,
but a cutoff of 90/225 (i.e. a dual of 2.5) is quite unusual - the
charge density would need a dual of 4 for a norm-conserving
pseudopotential (admittedly, once could use a bit less) but the ones
you are testing are not even norm-conserving, but PAW or ultrasoft,
where duals in the range 6-16 are common (for C and N, you might get
away with something like 6).
Generating pseudopotentials that are softer while remaining accurate
is exceedingly difficult, as you can see from the literature.
If I understood correctly the works you referred to, the delta
parameter seems much more large-scale (i.e. code to code) than the
scope I am working in, as I am not validating a pseudo generation
procedure, (which I am assuming to be already accurate, given the
source) but rather I am trying to understand the system-dependency of
the behavior of the pseudo. However, I am mistaken, I will be glad to
check those papers again and run some tests.
The delta parameter checks if the equation of state of an elemental
solid described with a pseudopotential is similar to the all electron
calculations of the same solid.
nic
--
Aldo Ugolotti, Ph.D.
Post-doc fellow
Materials Science Dept. U5,
Università degli Studi di Milano-Bicocca
via Cozzi 55,
20125 Milano (MI)
Italy
e-mail: [email protected]
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