[QE-users] Convergence to an AFM ground state

[David Guzman]

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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