[Wien] density matrix is not positive

[Fan]

According to you and Prof. Marks, I tried the AFM configuration for Nd,
everything went very well and it converged quickly even with finer converge
conditions, but 4f states still reside near Ef and still give a half-metal
solution. In fact, despite the existing literature, we synthesized this
compound ourselves, XRD and SEM show that it is the compound we are looking
for. However, it seems that it should be an insulator when close to 0K.
That is why I find the 4f states crossing Ef very troublesome. Yes, it is a
strange material, I guess I just have to go deeper.

Fan

On Sat, Jan 4, 2020 at 6:50 PM Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> 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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> >
>
> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
> WWW:
>
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