Dear Dave,

there are 2 issues that have to be considered when comparing the electronic bands of a primitive structure (e.g., primitive FCC) and of a (in principle equivalent) supercell built up from it (e.g., FCC represented by a simple cubic supercell containing 4 primitive FCC cells) :

1. The points in reciprocal space (k-points) where you wish to calculate the electronic bands are always given in exciting in coordinates which refers to the basis vectors in reciprocal space. In the example you mention, the coordinates of the X point which should be given in the input.xml file are
   • 0.5 0.5 0.0 for the primitive FCC cell.
   • 1.0 0.0 0.0 for the simple cubic supercell.
2. In the case of supercell calculations the size of the Brillouin zone is reduced, so that the energy bands appear to be folded back along the reduced directions. In your example, the (old, FCC) X point coincides with a traslation of the (new, simple-cubic supercell) Γ point. This is the reason why, if you calculate the energy bands for the simple cubic supercell at the X point of the primitive FCC structure, you get a list of energy bands which comprises the calculated values at both the X and Γ points of the FCC primitive lattice.

Best regards
Pasquale Pavone