cQED-iCIPT2: A Near-Exact Method for Polaritonic Chemistry

Abstract

Strong light-matter coupling in optical cavities provides a versatile platform for modulating chemical structure, reactivity, and spectroscopy, and hence motivates the development of ab initio cavity quantum electrodynamics (cQED) methods that can treat the electronic and photonic degrees of freedom on an equal footing. We present such a method, cQED-iCIPT2, by combining the near-exact iCIPT2 (iterative configuration interaction with selection and second-order perturbation theory) with the cQED Hamiltonian in two ways, cQED-PN-iCIPT2 and cQED-CS-iCIPT2. The former works directly in the photon-number representation, whereas the latter employs a coherent-state transformation that restores origin invariance of the cQED Hamiltonian and avoids artificially strong coupling in charged systems. To efficiently handle the tensor-product structure of the electron-photon wavefunction, we introduce a graded configuration space organized by photon numbers and decompose the key computational steps (selection, diagonalization, and perturbation correction) into intra- and inter-subspace steps, so as to maximize the reuse of the existing infrastructure in the MetaWave platform (J. Phys. Chem. A 2025, 129, 5170). The selection step enables automatic determination of the optimal number of photons without a priori truncation. The efficacy of the methods is showcased dissociation of N2, torsion of ethylene, proton transfer reactions in malonaldehyde and aminopropenal, and low-lying excited states of polyacenes. The results provide numerically accurate reference data and meanwhile reveal how the cavity can fine-tune reaction barriers, alter potential energy surfaces, and induce state crossings. As such, this work establishes a robust near-exact framework for polaritonic chemistry in the presence of both strong electron correlation and strong light-matter coupling.