[DFTB-Plus-User] Bulk/slab Fe optimization problems using pbc-0-3 or trans3d-0-1 parameters

[Bálint Aradi]

Dear Branislav

> I recompiled program and now I get reasonable structures.

That's a little bit worrisome, the results should not depend on the 
compilation on the program... If in doubt, try the official binaries via 
Conda, they should be OK.

> I tried two different setups for unit cell and got relatively good values:
> 
> ShellResolvedSCC = Yes
> ShellResolvedSpin = No
> 
> Lattice parameter = 2.9338 A and nearest neighbor distance = 2.54078 A
> 
> ShellResolvedSCC = No
> ShellResolvedSpin = Yes
> 
> Lattice parameter = 2.9393 A and nearest neighbor distance = 2.54553 A

Actually, I'd advise to set both, ShellResolvedScc as well as 
ShellResolvedSpin to "Yes".

> However, when I try to simulate 7x7x4 Fe(110) slab using LatticeOpt = 
> Yes and hexagonal symmetry I get quite distorted geometry.
> Do you think this is just a parameter set inherited problem or maybe 
> DFTB+ is only good when using cubic symmetry?

DFTB+ itself does not care about symmetry at all. The problem is, for 
sure, parameter set related. I know, that the iron in the pbc set was 
tested on small iron clusters.

https://doi.org/10.1016/j.chemphys.2004.03.034

I don't think, anybody ever checked or optimized it for slabs, so it 
might be, that the set is less optimal there. One would probably adjust 
the repulsive part, provided the band structure at the ab initio 
geometry is reasonable.

Best regards,

Bálint


-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-aradi/


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