We present an ab initio correlated approach to study molecules that interact strongly with quantum fields in an optical cavity. Quantum electrodynamics coupled cluster theory provides a nonperturbative description of cavity-induced effects in ground and excited states. Using this theory, we show how quantum fields can be used to manipulate charge transfer and photochemical properties of molecules. We propose a strategy to lift electronic degeneracies and induce modifications in the ground-state potential energy surface close to a conical intersection.5 MoreReceived 9 May 2020Revised 6 September 2020Accepted 14 October 2020DOI:https://doi.org/10.1103/PhysRevX.10.041043Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCavity quantum electrodynamicsChemical Physics & Physical ChemistryElectronic structureElectronic transitionsPolaritonsTechniquesAb initio calculationsCoupled clusterHartree-Fock methodsAtomic, Molecular & Optical