  We develop coupled-cluster theory for systems of electrons strongly coupled
to photons, providing a promising theoretical tool in polaritonic chemistry
with a perspective of application to all types of fermion-boson coupled
systems. We show benchmark results for model molecular Hamiltonians coupled to
cavity photons. By comparing to full configuration interaction results for
various ground-state properties and optical spectra, we demonstrate that our
method captures all key features present in the exact reference, including Rabi
splittings and multi-photon processes. Further, a path on how to incorporate
our bosonic extension of coupled-cluster theory into existing quantum chemistry
programs is given.
