[Wien] density matrix is not positive

[Peter Blaha]

The "new" (unmixed) dmat is always ok, only through mixing of the 
individual elements this may happen.

When reaching scf, the "mixed" dmat = "unmixed" dmat and thus this 
should no longer occur.

On 1/8/20 1:19 PM, pieper wrote:
> Sorry for interrupting, but I am intrigued. I was not aware that the 
> density matrix can leave the physically meaningfull realm of 
> semi-positive operators.
> 
> May I spoil this thread with the question why the mixer produces such 
> non-physical trial states? And if it does, what forces drive rho back to 
> proper density operators during convergence?
> 
> ---
> Dr. Martin Pieper
> Karl-Franzens University
> Institute of Physics
> Universitätsplatz 5
> A-8010 Graz
> Austria
> Tel.: +43-(0)316-380-8564
> 
> 
> Am 2020-01-04 11:50, schrieb 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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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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