Second order N-Electron Valence state Perturbation Theory for polaritonic systems
Molecules strongly coupled to optical cavities often show multi-configurational character, which the cavity can enhance by mixing electronic states. Describing them accurately requires a complete treatment of correlation between all the involved quantum particles. The available QED ab-initio methods do not treat it in its completeness except for QED-FCI. We extend second-order N-electron valence state perturbation theory (NEVPT2) to polaritonic chemistry, in its state-specific (QED-NEVPT2) and quasi degenerate (QED-QD-NEVPT2) forms. Here, the correlation due to the interaction of more degenerate configurations comes from a QED-CASSCF reference, while that associated with instantaneous interactions between the particles is added by second-order perturbation theory. Application of the methods to ammonia, ethylene and formaldimine revealed that this approach represents the perfect alternative to the well established QED-Coupled Cluster approach when the molecular system is described by more degenerate configurations. The approaches developed in this work have the potential to become state-of-the-art methodologies for the description of reactivity and photochemistry in polaritonic systems.