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

[Aldo Ugolotti]

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: a.ugolotti at campus.unimib.it


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: a.ugolotti at campus.unimib.it