QuantumATK Forum
QuantumATK => Questions and Answers => Topic started by: k.cerda on October 10, 2020, 11:09

Hi,
I'm just learning to use QuantumATK for my thesis work. I need to simulate organic molecules which contain 550015.000 atoms. Therefore DFT does not seem to be feasible due to the large number of atoms. I turned to SemiEmpirical method and it also does not seem to be capable of converging with such number of atoms. Today I was reading the QuantumATK manual pages and came across the following paragraph:
"Note
The HÃ¼ckel parameters have been fitted for nonselfconsistent calculations. To use the parameters in selfconsistent calculations, the selfconsistent onsite shifts must be compensated by a reverse shift of the vacuum_levels."
on page: https://docs.quantumatk.com/manual/SE.html (https://docs.quantumatk.com/manual/SE.html).
Combining this expression with the fact that the no SCF checkbox is checked by default in SemiEmpirical Calculator, can we say that the results of nonselfconsistent calculations with Semi Empirical Calculator in QuantumATK are valid and accurate? In other words, are the results of no SCF SemiEmpirical calculations valid? I understand that selfconsistent field is no doubt more accurate but given the very large number of atoms, can the results of nonselfconsistent Semi Empirical package be considered valid for a thesis or publication work?
Thanks,
K. Cerda

Dear Cerda,
I'm facing the same issue with my thesis, and I can't get any converged results with self consistent option using the semi empirical approach, did you reach to any result if we can rely on the non self consistent results? I've been searching for so long for any answer but was unable to find anything
Regards,

It's very hard to give a definite answer, but as you noted the Huckel model is actually designed to be nonselfconsistent, and is also originally developed for organic systems, so I think at the very least the results will be "representative" enough to use for real work. You can perhaps search the literature for other work on similar systems using the same model, and see how they treat SCF vs nonSCF.
Actually, if I recall correctly, we mainly added the SCF option for the semiempirical models to use them in transport simulations, where definitely the effect of charge transfer is crucial, and a nonSCF solution would not be accurate.
Btw, it is not entirely impossible to use DFT for the system size you mention, at least with a small basis set like SingleZetaPolarized. You might want to give it a try at least :)