[QE-users] Convergence to an AFM ground state

[Daniel Kaplan]

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
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