[Wien] The problem for atomic forces in the supercells of lanthanides compounds

[Peter Blaha]

You have to compare not just :FGL, but also other quantities like :MMI 
and :CTO and :QTL

Maybe, in your supercell of a surface it max want to do some 
reconstruction, which is only possible in the larger cell.

You may see some particular charge or magnetic (AFM ?) ordering and 
related to that forces indicating some reconstruction. You should also 
compare total energies. Is Etot more/less than 4x old cell ???  ....

In any case, remember, what L.Marks said:   plain GGA even with an open 
core approximation may not be sufficient, ....

Am 03.09.2021 um 17:40 schrieb Artem Tarasov:
> Dear wien2k users,
>
> Recently, I have noticed a strange problem related to the forces 
> inside the supercell for compounds containing 4f-atoms with 4f-states 
> placed on the core level ("open core approximation" for Tb, Dy, Ho, Ce 
> etc.). If we relax the slab of a cell with a period of 1x1 and a 
> symmetry group of P4mm, and then make a cell with a doubled lateral 
> period of 2x2 from the relaxed structure, then after converging the 
> density for the 2x2 cell the forces on all atoms in the supercell grow 
> significantly compared to the case of the 1x1 cell. Up to a jump from 
> 0.05 to 68 mRy/bohr on individual atoms. I thought that the problem 
> may be related to the shift of the k-grid, because when I move to the 
> supercell, I change the k-grid from 12x12 to 6x6, and I had supposed 
> that such a k-grid may not be enough for an accurate calculation. 
> However, even if I don't use the k-grid shift in both cases, this 
> problem persists. My calculation is a slab of CeB6 with symmetry P4mm 
> with Ce4f states given in case. inc as
>
> 16 0.70  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> …
> 4, 3,0.43               ( N,KAPPA,OCCUP)
> 4,-4,0.57               ( N,KAPPA,OCCUP)
>
> Thank you in advance for your help.
>
> Best regards,
> Artem Tarasov
> Department of Solid State Electronics
> Saint Petersburg University
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Peter Blaha,  Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-158801165300
Email: peter.blaha at tuwien.ac.at
WWW:   http://www.imc.tuwien.ac.at      WIEN2k: http://www.wien2k.at
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