[DFTB-Plus-User] Correctness of SCC convergence in DFT+NEGF

[anz118466 at cse.iitd.ac.in]

Dear all,

I have simulated two testcases on the DFTB+NEGF. Both consist of a
biomolecule on a GNR. I observed two very different trends in SCC
convergence while simulating for the transmission spectra, which I am
sharing here.

The first case consists of 1795 atoms in total and saw an extremely fast
convergence in just 14 steps. The second one, however, had 604 atoms and
saw convergence in around 600+ steps. Also, the SCC error in the second
case increased gradually first and then decreased rapidly. For example,
the SCC error reached a high of 0.21776430E+03 in 402 steps. It then
gradually decreased to 0.31423479E+02 in another 200 steps, and fell
sharply to 0.15680539E-05 in another 5 steps. The value of 'SCCTolerance'
was kept 1e-4 in both the cases.

Looking at these two trends, I am confused with the correctness of the
final output. How do I ensure that no numerical noise has crept in any of
my two testcases? Also, I observe that the final output changes by
modifying the poisson box dimensions. Is there any rule of thumb regarding
the choice of the poisson box? Please let me know if you need more data
from me for further clarification.

Thanks & Regards,
Anusha Iyer