Dear all,

eventually, setting up a spin-polarized calculation solves the problem. I attach the resulting band-structure.
The disadvantage of this approach is that I had to impose a spin-polarized occupation for the carbon atoms in the inp.xml. While self-consistency brings the system towards a non-magnetic solution, the final net magnetic moment is not exactly zero but of the order of 8x10-7...

Parsing the output file, I still have one little question. Equal and opposite charges have been placed on the external sheets 1 and 2. Why does the code reports external electric fields of opposite magnitude on these two planes?

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parameters for external electric field:
total electronic charge = 12.00000
total nuclear charge = 12.00000
z-z1 of external sheet = 0.00000 a0 = 0.00000 A
charge on external sheet 1 = 0.00500 (surface density= 0.00027 e/a.u.**2)
external field on sheet 1 = -0.17264E+08 V/cm
charge on external sheet 2 = -0.00500 (surface density= -0.00027 e/a.u.**2)
external field on sheet 2 = 0.17264E+08 V/cm
NOTE: The equation for the E field assumes two oppositely charged plates.
You may need to divide by two before summing the external fields to avoid double counting
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I guess that in this case, the actual field within the capacitor is of 0.17264E+08 V/cm, which is consistent with the computed band-structure. Is that right?

Best regards,

Simon