Enabling Polaritonic Electronic Structure Theory for Psi4: The psi4-cbo package

Abstract

We present psi4-cbo, an open-source extension of the Psi4 program for simulations within the cavity Born-Oppenheimer Approximation (CBOA). By treating the cavity photon mode on the same footing as the nuclear coordinates, \texttt{psi4-cbo} incorporates light-matter coupling into standard electronic structure workflows. These workflows include self-consistent field calculations, response theory, analytic gradients, joint Hessians, and geometry optimization. The package is designed for reproducibility and is documented with working examples for its main workflows. To evaluate the approximations inherent to the CBOA framework, we use an exact \gls{qedfci} reference. Resolving the correlation energy into electron-electron and electron-photon channels enables us to precisely determine the point at which the CBOA becomes quantitatively sufficient. Additionally, we highlight that, since self-consistent treatments dress the cavity modes, satisfying experimental resonance conditions is method-dependent. As a practical application, we investigate the chromophore acrolein under vibrational strong coupling and detail how to locate resonances on the dressed photon modes rather than the bare molecular bands.