[DFTB-Plus-User] bcc Fe in DFTB+

[Bálint Aradi]

Dear Peter,

a few notes on your query about Fe:

* The iron parameters in pbc were fitted and tested on iron clusters, so 
bulk might be OK, but doesn't necessarily have to be. (A lattice 
constant difference of 3% is indeed not really good, but I'd still 
consider it as acceptable when using DFTB.)

* I'd suggest to use shell resolved calculation for charges as well 
(ShellResolvedScc = Yes), as the original work used shell resolved scc 
as well.

* The order of your spin constants is OK, but take the PBE values (you 
seem to use the LDA values). As the integrals in the pbc set were 
created with PBE, that would be more consistent (although probably does 
not make a big difference).

Best regards,

Bálint


On 3/4/26 10:04 PM, Peter Klaver wrote:
> Hello people,
> 
> I wanted to see how well DFTB+ can reproduce DFT results for bcc Fe with 
> some self-interstitial atoms in the lattice (radiation damage). I 
> started with defect-free Fe. For bulk bcc Fe I find a magnetic moment of 
> 2.13 Bohr magneton, which is quite good, but a lattice parameter of 
> 2.942 Å, where experimental data extrapolated to 0 K gives somewhere 
> halfway between 2.84-2.85 Å. So it’s 3.4% off. For an earlier use of 
> DFTB+, on graphene and graphane, I found bond lengths were usually less 
> than 1% different from DFT and still less than 2% for the worst cases.
> 
> The README with the pbc Slater-Koster parameterisation files says “The 
> iron parameters have been applied to structural and magnetic properties 
> of iron clusters [Iron], but should also give reasonable results for bcc 
> bulk iron.” I wonder: is the 3.4% deviation in lattice parameter 
> reasonable, or is it an indication that I might be doing something wrong?
> 
> I sought for DFTB+ papers on bcc Fe, but couldn’t find much. Has anyone 
> here used DFTB+ for bcc Fe, and if so, what were your findings as to how 
> well the pbc Slater-Koster files reproduce bcc Fe properties, in 
> particular the lattice spacing?
> 
> One point where, as a relatively inexperienced DFTB+ user, I wasn’t too 
> sure if I did things right was in the spin constants. I put in
> 
> SpinConstants = {
> ShellResolvedSpin = Yes
> Fe = {
> -0.013 -0.009 -0.003 -0.009 -0.011 -0.001 -0.003 -0.001 -0.015
> }
> }
> 
> If anyone is working on bcc Fe, did you put in a similar line for that?
> 
> Kind regards,
> Peter
> 
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-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.uni-bremen.de/en/lmcqm/b-aradi

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