  We present a new quantum embedding theory called dynamical configuration
interaction (DCI) that combines wave function and Green's function theories.
DCI captures static correlation in a correlated subspace with configuration
interaction and couples to high-energy, dynamic correlation outside the
subspace with many-body perturbation theory based on Green's functions. In the
correlated subspace, we use a wave function description to avoid embedding the
two-particle vertex, which greatly simplifies the frequency structure of the
embedding. DCI takes the strengths of both theories to balance static and
dynamic correlation in a single, fully ab-initio embedding concept. We show
that treating high-energy correlation up to the $GW$ and Bethe-Salpeter
equation level is sufficient even for challenging multi-reference problems. Our
theory treats ground and excited states on equal footing, and we compute the
dissociation curve of N$_2$, vertical excitation energies of N$_2$ and C$_2$,
and the ionization spectrum of benzene in excellent agreement with high level
quantum chemistry methods and experiment.
