[QE-users] Convergence to an AFM ground state

[Paolo Giannozzi]

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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