[QE-users] Convergence to an AFM ground state

[Daniel Kaplan]

Hello Paolo and David.
I tested the exact same system with ecutrho=4,6,8,10*ecutwfc to no avail.
Perhaps there's something else still missing.
Yours,
Daniel


On Sun, Feb 9, 2020, 20:05 David Guzman <davgumo at me.com> wrote:

> Paolo,
> Thanks for the comment. Even with appropriately chosen cutoffs for the
> kinetic energy and charge density, the AFM solution is difficult to
> converge with PAW pseudopotentials (suggested from website).
> The system I was trying (FeSb2) is less complicated than Daniel’s system.
> I thought it was an isolated case, but now that Daniel is also having
> problems maybe there’s more to it.
>
> Regards,
> David Guzman
> Brookhaven National Laboratory
>
> On Feb 9, 2020, at 12:38 PM, Paolo Giannozzi <p.giannozzi at gmail.com>
> wrote:
>
> 
> On Sun, Feb 9, 2020 at 3:36 PM David Guzman <davgumo at me.com> wrote:
>
>> Not sure [...] if there are extra setting that should go along with those
>> potentials.
>>
>
> the only extra setting with USPP and PAW is that you may (and often, you
> should) use a cutoff for the charge density (ecutrho), that differs from
> 4*ecutwfc (default value).
>
> Paolo
>
>
>
>>
>> On Feb 9, 2020, at 9:10 AM, Daniel Kaplan <danielkaplan137 at gmail.com>
>> wrote:
>>
>> 
>> Hello All!
>>
>> I'm trying to calculate a system with a *known* AFM ground state. In the
>> attached example, I provide the input data I'm using for CuMnAs -- a
>> tetragonal anti-ferromagnet.
>> I've started on this project by first executing the examples, and
>> particularly FeO.
>> I tested this example against all sorts of variations: different
>> functionals, with/without U, and so on. Without any further tweaking, the
>> system always converged to the AFM ground-state, provided the initial
>> moments were also oriented in the AFM configuration.
>>
>> Which makes my failure in this (CuMnAs) system even more puzzling.
>> Firstly, *without* any constraints, the system does not converge to an
>> AFM state.
>> 1. Using 'constrained_magnetization=total' leads to completely wrong
>> results, with a divergent "Magnetic field".
>> 2. A more-or-less sensible result can be obtained with
>> 'constrained_magnetization='atomic' (as shown), however, the resultant
>> magnetization is not altogether anti-ferromagnetic. Note that the system is
>> in general endowed with PT-symmetry. The resultant eigenvalues *DO NOT* show
>> this and you can also see the disparity in the magnetic moments of the Mn
>> atoms, as well as eigenvalue difference of more than 1meV for some bands
>> and k-points.
>> 3. This behavior is *weakly* dependent on lambda. I tried fiddling
>> around with the values. A certain increase worsens the results, then seems
>> to improve it, only to worsen again. What is a reason value for the
>> constraint, in your estimation? I take it to be 5% of the unperturbed
>> energy (i.e., energy without the constraint).
>> 4. Testing the *exact* same system on different software (VASP, in this
>> case), converged very well to the AFM state (i.e., PT symmetry was
>> recovered to less than 1meV).
>>
>> What am I doing wrong, therefore?
>> I would appreciate any advice.
>> Yours thankful,
>> Daniel Kaplan
>> Dept. of Condensed Matter Physics
>> Weizmann Institute of Science
>> <scf.in>
>> <scf.out>
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>
>
> --
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
> _______________________________________________
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