Subsystem Density-Functional
Theory and its extension to excited
states, namely, subsystem Time-Dependent Density-Functional Theory,
have been proven to be efficient and accurate fragmentation approaches
for ground and excited states. In the present study we extend this
approach to the subsystem-based description of total systems by means
of GW and the Bethe–Salpeter equation (BSE). For this, we derive
the working equations starting from a subsystem-based partitioning
of the screened-Coulomb interaction for an arbitrary number of subsystems.
Making use of certain approximations, we develop a parameter-free
approach in which environmental screening contributions are effectively
included for each subsystem. We demonstrate the applicability of these
approximations by comparing quasi-particle energies and excitation
energies from subsystem-based GW/BSE calculations to the supermolecular
reference. Furthermore, we demonstrate the computational efficiency
and the usefulness of this method for the description of photoinduced
processes in complex chemical environments.