[Wien] density matrix is not positive

Mon Jan 6 02:27:12 CET 2020

 I don't understand why your compositions should be an insulator:
neither Nd4Ni3O8
nor Nd4Ni3O10 match to a "nice" valence! Did you check the BVS (grep Bond
case.outputnn) ?

What does match is Nd4Ni3O9, which would be reasonable Nd3+, Ni2+and O2-.

N.B., how well did you calibrate your EDX for the oxygen content -- hard to
be accurate unless you have a windowless detected or used a microprobe
instead. XRD can be dodgy for the O content, you can check the temperature
factors and also allow the O occupancy to vary.

On Sun, Jan 5, 2020 at 7:03 PM Fan <fxinwei123 at gmail.com> wrote:

> 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
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-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu
Corrosion in 4D: www.numis.northwestern.edu/MURI
Co-Editor, Acta Cryst A
"Research is to see what everybody else has seen, and to think what nobody
else has thought"
Albert Szent-Gyorgi
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