[DFTB-Plus-User] Spin-polarized md simulation using DFTB+

[Dhungana, Kamal B]

I would like to perform a spin-polarized md-simulation for a system containing among other things Sulfur atoms using the DFTB+ package.


I have a few doubts that I would very much appreciate if you could help me resolve:


1) The basis set for Sulfur atom in DFTB3 includes d-orbitals. I have read in the literature (

"Zoltan Bodrog* and Balint Aradi Phys. Status Solidi B 249, No. 2, 259–269 (2012) / DOI 10.1002/pssb.201100524) the following: "The simplistic, ‘semidiagonal’ U matrix is a good approximation for elements up to the third period, but it is significantly wrong
above (subshell hardness errors going well beyond 10%, see
Table 1) if d orbitals are present in the atomic basis."

I understand that S is not third period but certainly polarizable. Is it OK to therefore use an atomic based Hubbard matrix in this particular case for open shell situations for example including an excess electron? In other words, since I would like to use DFTB3  and the 3ob-3-1 S-K parameters and orbital resolved Hubbard derivatives are not available for most of the atoms is it reasonable to use the atomically-resolved methodology with the option ShellResolvedSpin=Yes in DFTB+? I notice that the spin parameters exist for all the orbitals in my system.


2) My second question is related to Wpp vs Wdd in this particular case. In

Patrick Melix, Augusto Faria Oliveira, Robert Rüger, Thomas Heine, Theor Chem Acc (2016) 135:232 spin polarized SCC-DFTB using 3ob-3-1 is used. Only one spin parameter is provided per atom. If I look at this spin parameter in the case of the S atom it appears to be that corresponding to Wpp. However, 3ob-3-1 includes d orbitals for S.


How should I understand this (beyond the fact that d orbitals are in principle empty but are there for polarization effects)? How should one decide about using Wpp or Wdd in this case, particularly considering that the condensed phase system includes an excess electron (not necessarily localized on S but who knows)?


Sorry that my email is verbose, I am trying to understand as much as I can how to properly use DFTB without trivial mistakes. I have found almost no calculations of open shell systems in the literature and I would like to know if this is for some good reason.


Best Regards,

Kamal
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