Strong Coupling Quantum Electrodynamics Hartree–Fock Response Theory

The development of reliable <i>ab initio</i> methods for light-matter strong coupling is necessary for a deeper understanding of molecular polaritons. The recently developed strong coupling quantum electrodynamics Hartree-Fock model (SC-QED-HF) provides cavity-consistent molecular orbitals, overcoming several difficulties related to the simpler QED-HF wave function. In this paper, we further develop this method by implementing the response theory for SC-QED-HF. We compare the derived linear response equations with the time-dependent QED-HF theory and discuss the validity of equivalence relations connecting matter and electromagnetic observables. Our results show that electron-photon correlation induces an excitation redshift compared to the time-dependent QED-HF energies, and we discuss the effect of the dipole self-energy on the ground and excited state properties with different basis sets.