[QE-users] [QE users] pseudopotential hardness and transferability

[Nicola Marzari]

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


-- 
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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