[DFTB-Plus-User] How accurate is DFTB+ in reproducing DFT result?

[Hemanth Haridas]

I am trying to study the shifts in the Fermi level of a graphene - ssDNA
hydrid system, depending on the identity of the ssDNA. The structures are
obtained from a classical MD simulation, and cannot be geometry optimized.
I ran two distinct calculations: one at the gamma point and other one at a
Kpoint mesh of 10x10x1. From the PDoS plots, what I observe is that the
Fermi of the system is dominated by the contributions from graphene (which
is expected). However, I also find that the HOMO of the ssDNA molecules
lies very close to the Fermi of the system. When I am trying to compare my
results to this paper
<https://pubs.acs.org/doi/pdf/10.1021/jp402403f?src=getftr>, I see that
they get an appreciable difference in the HOMO levels of the nucleobases.
The interactions between the ssDNA and the graphene surface would be
largely pi-stacking interactions. This leads to my question, How accurate
are the results from DFTB+ calculations, to be compared with ab-initio DFT
calculations. Any comments on this would be much appreciated.

-- 
*Hemanth H 18310019*
Research Scholar
IIT Gandhinagar
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