If we have a heterojunction device supercell (i.e. two different materials A aand B junction at the middle of the central "Device region") as in
http://quantumwise.com/support/tutorials/item/94-carbon-nanotube-junctionsand make a device from Bulk config/ molecular config, what determines whether the electrodes will be considered as periodic? (i.e. is it necessary to have the length of the electrode region to be physically much longer than the central region or vice versa to assume ideal contacts)
Before making the device (i.e. the bulk config) if we repeat the structure in C it becomes A-B-A-B-A-B, what ensures that when we make a device configuration, that it is actually a A-B heterojunction we are simulating.
I just want to know how ATK handles this internally and makes the choice?