  Multi-scale computational approaches are important for studies of novel,
low-dimensional electronic devices since they are able to capture the different
length-scales involved in the device operation, and at the same time describe
critical parts such as surfaces, defects, interfaces, gates, and applied bias,
on a atomistic, quantum-chemical level. Here we present a multi-scale method
which enables calculations of electronic currents in two-dimensional devices
larger than 100 nm$^2$, where multiple perturbed regions described by density
functional theory (DFT) are embedded into an extended unperturbed region
described by a DFT-parametrized tight-binding model. We explain the details of
the method, provide examples, and point out the main challenges regarding its
practical implementation. Finally we apply it to study current propagation in
pristine, defected and nanoporous graphene devices, injected by chemically
accurate contacts simulating scanning tunneling microscopy probes.
