  We present a fully ab initio approach based on many-body perturbation theory
in the GW approximation, to compute the quasiparticle levels of large interface
systems without significant covalent interactions between the different
components of the interface. The only assumption in our approach is that the
polarizability matrix (chi) of the interface can be given by the sum of the
polarizability matrices of individual components of the interface. We show
analytically, using a two-state hybridized model, that this assumption is valid
even in the presence of interface hybridization to form bonding and
anti-bonding states, up to first order in the overlap matrix elements involved
in the hybridization. We validate our approach by showing that the band
structure obtained in our method is almost identical to that obtained using a
regular GW calculation for bilayer black phosphorus, where interlayer
hybridization is significant. Significant savings in computational time and
memory are obtained by computing chi only for the smallest sub-unit cell of
each component, and expanding (unfolding) the chi matrix to that in the unit
cell of the interface. To treat interface hybridization, the full wavefunctions
of the interface are used in computing the self-energy. We thus call the method
XAF-GW (X: eXpand-chi, A: Add-chi, F: Full wavefunctions). Compared to
GW-embedding type approaches in the literature, the XAF-GW approach is not
limited to specific screening environments or to non-hybridized interface
systems. XAF-GW can also be applied to systems with different dimensionalities,
as well as to Moire superlattices such as in twisted bilayers. We illustrate
the generality and usefulness of our approach by applying it to self-assembled
PTCDA monolayers on Au(111) and Ag(111), and PTCDA monolayers on
graphite-supported monolayer WSe2, where good agreement with experiment is
obtained.
