[Wien] density matrix is not positive

[Peter Blaha]

I repeated your calculations. Yes, intermediately I also got these 
density matrix warnings, but NOT with final convergence.

Most likely, this warning should be removed from mixer (Laurence !), as 
it does not give any meaningful hint or it should be printed only if one 
is nearly converged (:DIS is lower than ...)

I have a few other comments:

You seem to start a Ferro-magnetic calculation. As was mentioned before, 
maybe an AFM structure is more stable. So check other possible magnetic 
configurations.

You seem to start with init_lapw, and thus with the default 
(ferromagnetic) case.inst

For magnetic oxides it is highly recommended to start with:

instgen -ask
This lets you define a possible AFM arrangement and in particular you 
should set the O-atoms as non-magnetic. This save a lot of scf cycles.

-rkmax 7.5 is quite some "overkill". This rkmax value is for the small 
O-atoms and RKmax=6 (beginning) and 7 (final convergence) is sufficient.

On the other hand, a HDLO for Nd-4f states is highly recommended.

I never start the scf-cycle with "-orb" right after intitialization, but 
always do (at least a crude) GGA calculation before switching on -orb. 
Otherwise you may even more likely end up in some local minimum but not 
the ground state.

I got a half-metallic solution with a spin-up 4f peak down at -3.5 eV 
and another one at EF. You expected that the 4f states are all removed 
from EF, but your compound seems to be "special" (is this compound 
experimentally known ??).

The stoichiometry of Nd4 Ni3 O8  suggests Ni2+ ions, leaving a (Nd4)10+
charge state, i.e. a Nd2.5+ state, which is very unusual. But I got 34f 
electrons, which hints to a Nd3+ and in fact one of your Oxygens has 
p-states above EF.

With this half-filled 4f-DOS peak at EF also spin-orbit could be very 
important.

And as always in such LDA+U calculations, play with the initial 
case.dmat file. In my case I have 2 4f orbitals "fully" occupied (and 
thus shifted down), but 2 others are partly occupied (close to 1/2) and 
thus not much shifted and pinned to EF.

As mentioned before, additional symmetry splitting with AFM for the 2 
equivalent Nd atoms may lead to further splitting.


Am 03.01.2020 um 10:30 schrieb Fan:
> Thank you for your reply, Prof. Blaha. In fact, when I change parameters 
> in inorb, I always do 'rm case.dmat*' and 'rm case.vorb*' , so I think 
> it should not be the problem. But I do value your suggestions, so I 
> started a new session, only used the struct file of Nd4Ni3O8 which is 
> attached here, and then
> init_lapw -b -rkmax 7.5 -numk 2000 -sp -vxc 5 -lvns 6
> create inorb and indm by copying them from templates and modify them 
> accordingly
> runsp_lapw -p -orb -i 200 -ec 0.0001 -cc 0.001 -NI
> during the scf cycle, QTL-B warning appeared, but the value was very 
> small (2~3, still could be a trouble?), the density matrix warnings 
> appeared at about the 20th cycle and persisted.
> 
> With regards.
> 
> Fan
> 
> 
> 
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-- 
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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