  The quasiparticle self-consistent QS$GW$ approach incorporates the
corrections of the quasiparticle energies from their Kohn-Sham density
functional theory (DFT) eigenvalues by means of an energy independent and
Hermitian self-energy matrix usually given in the basis set of the DFT
eigenstates. By expanding these into an atom-centered basis set (specifically
here the linearized muffin-tin orbitals) a real space representation of the
self-energy corrections becomes possible. We show that this representation is
relatively short-ranged. This offers new opportunities to construct the
self-energy of a complex system from parts of the system by a cut-and-paste
method. Specifically for a point defect, represented in a large supercell, the
self-eneregy can be constructed from those of the host and a smaller defect
containing cell. The self-energy of the periodic host can be constructed simply
from a $GW$ calculation for the primitive cell. We show for the case of the
As$_\mathrm{Ga}$ in GaAs that the defect part can already be well represented
by a minimal 8 atom cell and allows us to construct the self-energy for a 64
cell in good agreement with direct QS$GW$ calculations for the large cell.
Using this approach to an even larger 216 atom cell shows the defect band
approaches an isolated defect level. The calculations also allow to identify a
second defect band which appears as a resonance near the conduction band
minimum. The results on the extracted defect levels agree well with Green's
function calculations for an isolated defect and with experimental data.
