[QE-users] Specific energy of the system increases with increase of supercell. Why?

[Evgeny Permyakov]

I use wide-spread ribbon model for MoS2 edge. To ensure correctness of
the model I calculated supercells with 2,3,4,6 periods (Mo atoms)
along the edge. I expected that at least for one row of 2-4-6 or 3-6 I
would see constant specific energy per edge Mo atom.

Instead I see a slight, but constant increase in specific energy. The
increase is small (~0.2 eV/period) but constant. It was observed for
series with 2k points for all supercell sizes and for series where I
used such number of K-point that product of supercell size and number
of k-points were preserved.  I could dismiss it, but ! That increase
is observed for some structures in same model and not observed for
others. If the increase was uniform between structures, I could
dismiss it. As of now I need to understand where it comes from. What I
should see for in my results and what could be the reason?

Interestingly, when I run same calculation with Gamma-point, I saw
different pattern, but this is of no importance now.