Does central region periodic?

[tigeryzz]

As we known, the electrode is periodic at x,y directions, Dose central region (I mean the molecule between two electrode ,including the surface of electrod) periodic, so that we need to enlarge the electrode to make there is no coupling between molecules in the nearest unit cells?

[Anders Blom]

The entire system is periodic in the XY directions, and there is, ultimately, only one unit cell in XY, common for the electrodes and the central region. So, yes, you need to account for interactions between neighboring cells in the central region, already when making the electrode unit cell.