[Wien] density matrix is not positive

[pieper]

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
>> 
>> 
>> 
>> _______________________________________________
>> Wien mailing list
>> Wien at zeus.theochem.tuwien.ac.at
>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>> SEARCH the MAILING-LIST at:  
>> http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
>>