[Wien] Problems with convergence with LDA+U calculations including spin-orbit coupling

[Peter Blaha]

The convergence behaviour is not very good, but it is not too bad either.

Check also :MMT or some :MMI0xx

For more complicated cases (metallic ??) it is well possible that one needs more
than 40 iterations. Thus my suggestion is to keep going and submit another job.

Sometimes it is better to keep the history (use -NI switch), sometimes it can also
be better to remove the history (rm *.broy*; which will be done automatically when
-NI is not supplied).

The mixing parameter has little importance with MSEC1, but still, a too small mixing
can lead to too small steps and thus "pseudoconvergence" (this simply means, that so
little new density is mixed, that eg. the total energy is not changing and one thinks
the calculation is converged).

PRATT is most likely "useless" for your problem.

In LDA+U calculations it is easily possible to reach different states (different
orbital occupations), which are stable and local minima. Once you have reached
convergence, there is never a guarantee that this is the groundstate and really
has the lowest energy.
The suggested procedure in the UG is NOT ALWAYS leading to the lowest state,
but often it will do.


> I have searched the list but I'm a bit confused with the posts, so I'll pose my questions here.
>
> I am running LDA+U with and without spin-orbit coupling for a structure in a cell with the following cell parameters (Bohr):
>
> P   LATTICE,NONEQUIV.ATOMS: 14127_P4/mbm
> MODE OF CALC=RELA
>   10.353171 10.353171 43.972470 90.000000 90.000000 90.000000
>
> and a cell sampling of 12x12x3, which gives 432 points in the BZ, 42 in the IBZ. My RKMAX is 8 - it was necessary to go to such high values to converge the spin moment of
> the system, as well as the energy down to 1 mRy. I followed the procedure stated in the manual: started with the LSDA calculation, then included spin-orbit coupling and now
> I am including the U gradually (well, at this point, just trying to...). I have to get to U=4 eV, so I'm growing it in steps of 1 eV.
>
> Convergence was very good for the LSDA+U calculations without SOC, and they have finished, already (that is, I have already gotten to the 4 eV in U). However, I am having
> problems converging the scf, with the charge and energy distances (as obtained by grep convergence case.dayfile) oscillating, and never attaining convergence, which I set
> to ec=0.00001 and cc=0.0001 (the numbers are appended by the end of this message). More than 40 SCF steps have passed.
>
> Usually, in pseudopotential calculations, one possible solution is to decrease the mixing weight of the charge densities for the following SCF step, and the other is to
> increase the smearing temperature. I would like to tweak with the mixing rather than increasing the temperature, if possible, because from my experience, increasing the
> temperature can sometimes lead to a non-magnetic state. However, I see from the mailing list that (at least in LAPW calculations), too small a mixing can lead to
> pseudo-convergence - which I don't understand what is. So, my questions are the following:
>
> 1) What is the best thing to do to make my calculation converge: increase the mixing, or to diminish it? Should I change the mixing for MSEC1, and if so, by which amount?
> Or, would it be good to change to PRATT, instead? I am quite lost with regards to that.
> 2) Would the application of SOC to the already converged LSDA+U calculation yield too different results from starting from a converged LSDA+SOC with later inclusion of U?
> 3) What is the pseudo-convergence mentioned in previous emails in the list?
>
> Of course, I will gladly provide more information, if needed.
>
> Best regards,
>
> Marcos
> Universidad de Cantabria, Spain
>
> =======================================
>
> Convergence of energy and charge distances:
>
> :ENERGY convergence:  0 0.00001 .0002941150000000
> :CHARGE convergence:  0 0.0001 .0088826
> :ENERGY convergence:  0 0.00001 .0000916950000000
> :CHARGE convergence:  0 0.0001 .0085946
> :ENERGY convergence:  0 0.00001 .0000428400000000
> :CHARGE convergence:  0 0.0001 .0112915
> :ENERGY convergence:  0 0.00001 .0001772500000000
> :CHARGE convergence:  0 0.0001 .0113780
> :ENERGY convergence:  0 0.00001 .0002456850000000
> :CHARGE convergence:  0 0.0001 .0104415
> :ENERGY convergence:  0 0.00001 .0001420300000000
> :CHARGE convergence:  0 0.0001 .0083852
> :ENERGY convergence:  0 0.00001 .0000500150000000
> :CHARGE convergence:  0 0.0001 .0084675
> :ENERGY convergence:  0 0.00001 .0001809700000000
> :CHARGE convergence:  0 0.0001 .0134776
> :ENERGY convergence:  0 0.00001 .0001993400000000
> :CHARGE convergence:  0 0.0001 .0112308
> :ENERGY convergence:  0 0.00001 .0002392700000000
> :CHARGE convergence:  0 0.0001 .0104208
> :ENERGY convergence:  0 0.00001 .0002088150000000
> :CHARGE convergence:  0 0.0001 .0069138
> :ENERGY convergence:  0 0.00001 .0001630050000000
> :CHARGE convergence:  0 0.0001 .0109876
> :ENERGY convergence:  0 0.00001 .0002176700000000
> :CHARGE convergence:  0 0.0001 .0012609
> :ENERGY convergence:  0 0.00001 .0001806300000000
> :CHARGE convergence:  0 0.0001 .0042825
> :ENERGY convergence:  0 0.00001 .0000720450000000
> :CHARGE convergence:  0 0.0001 .0045301
> :ENERGY convergence:  0 0.00001 .0000129050000000
> :CHARGE convergence:  0 0.0001 .0011304
> :ENERGY convergence:  0 0.00001 .0002283150000000
> :CHARGE convergence:  0 0.0001 .0047456
> :ENERGY convergence:  0 0.00001 .0003098100000000
> :CHARGE convergence:  0 0.0001 .0100820
> :ENERGY convergence:  0 0.00001 .0003708750000000
> :CHARGE convergence:  0 0.0001 .0087276
> :ENERGY convergence:  0 0.00001 .0000788950000000
> :CHARGE convergence:  0 0.0001 .0090876
> :ENERGY convergence:  0 0.00001 .0000849050000000
> :CHARGE convergence:  0 0.0001 .0093747
> :ENERGY convergence:  0 0.00001 .0000842350000000
> :CHARGE convergence:  0 0.0001 .0079561
> :ENERGY convergence:  0 0.00001 .0000553500000000
> :CHARGE convergence:  0 0.0001 .0122027
> :ENERGY convergence:  0 0.00001 .0000434050000000
> :CHARGE convergence:  0 0.0001 .0109352
> :ENERGY convergence:  0 0.00001 .0000227750000000
> :CHARGE convergence:  0 0.0001 .0122136
> :ENERGY convergence:  0 0.00001 .0000194300000000
> :CHARGE convergence:  0 0.0001 .0121261
> :ENERGY convergence:  0 0.00001 .0000292700000000
> :CHARGE convergence:  0 0.0001 .0131197
> :ENERGY convergence:  0 0.00001 .0000360650000000
> :CHARGE convergence:  0 0.0001 .0110031
> :ENERGY convergence:  0 0.00001 .0000300950000000
> :CHARGE convergence:  0 0.0001 .0115123
> :ENERGY convergence:  0 0.00001 .0000323600000000
> :CHARGE convergence:  0 0.0001 .0099887
> :ENERGY convergence:  0 0.00001 .0000147550000000
> :CHARGE convergence:  0 0.0001 .0110331
> :ENERGY convergence:  0 0.00001 .0001196200000000
> :CHARGE convergence:  0 0.0001 .0005253
>
>
>
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-- 
-----------------------------------------
Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
-----------------------------------------