Convergence issues

[AsifShah]

I see in some paper they use surface energy convergence to check how many layers of active material is needed for proper calculations however when electrode convergence is required they use LDOS, local density of states to check after how many layers it is not changing.
My question is why LDOS is used for electrode convergence and surface energy for active material?

[Petr Khomyakov]

You might take a look at this technical notes on device calculation convergence https://docs.quantumatk.com/technicalnotes/negf_convergence_guide/negf_convergence_guide.html.

Having flat LDOS in the electrode extension region ensures that the device central region is large enough, compared to the screening length of the system.

I guess adding more surface layers might serve the same purpose or have a different one, e.g., to check if the subsequent geometry relaxation of the near surface region capture the local strain accurately; note that electrode extension regions are fixed (not allowed to relax by construction). Probably, one should consider every specific case to understand the actual purpose.