A multireference coupled cluster theory based on quantum electrodynamics: Physical aspects of open-shell treatment

An electrodynamical coupled cluster (CC) formulation employing standard QED Hamiltonian that is written in Coulomb gauge and uses a multiconfiguration SCF picture of matter fields is discussed here. It employs a radiative cluster, matter clusters and their pair modifications, and considers a number state distribution of photons in thermal equilibrium. QED and electron correlation through CC are treated on the same footing. Thus an averaging over the radiation state is done to generate Lamb, Breit and hyperfine interactions from the radiative cluster. Because of the thermal distribution, it leaves a residual transverse interaction that may modify the static correlation. Dynamical correlation effects are determined next while using the exponential matter cluster in traditional ways of multireference CC. Deexcitations to negative-energy levels, included in matter clusters, generate vacuum polarization effects. The dynamical correlation energy includes relativistic corrections as well as QED contributions, namely, Lamb, Breit, hyperfine and pair energy.