The difference of 15 eV is of the total energy, not per atom. The number of atoms are the same for periodic and Green's function calculations. The reason I want to do this comparison is curiosity. I am learning the Green's function method, and I am wondering on what extent does the Green's function method is equivalent to periodic calculations.

In principle, these two calculations shall be equivalent, so the difference in total energy must result from the interface between leads and the scattering region. So, could you please provide more details on how QuantumATK dealing with the lead/scattering region interface?

Best,

Yun-Peng