Linear-response quantum-electrodynamical density functional theory based on two-component X2C Hamiltonians

Linear-response quantum electrodynamical density functional theory (QEDFT) enables the description of molecular spectra under strong coupling to quantized photonic modes, such as those in optical cavities. Recently, this approach was extended to the relativistic domain using the four-component Dirac–Coulomb Hamiltonian. To provide a computationally efficient yet accurate alternative—particularly for modeling 2D spectra or collective coupling for large, heavy-element systems—this article introduces a two-component linear-response QEDFT method based on exact two-component (X2C) Hamiltonian models. We derive how the parent four-component Hamiltonian for coupled electron–photon systems undergoes the X2C transformation. Moreover, we show that, under common weak-field and dipole approximations, it suffices to apply the X2C transformation only during the ground-state self-consistent field procedure, with the subsequent calculations performed fully in the two-component regime using the same X2C decoupling matrix. The current implementation includes the atomic mean-field (amfX2C), extended atomic mean-field (eamfX2C), and molecular mean-field (mmfX2C) Hamiltonian models. Benchmark calculations demonstrate that the X2C approach closely reproduces reference four-component results, enabling us to efficiently tackle systems that would be otherwise computationally too expensive. As applications, we compute 2D spectra of a mercury porphyrin complex in a Fabry–Pérot cavity, demonstrating off-resonant coupling and the appearance of multiple polaritonic branches. We also study a chain of AuH molecules, showing that collective coupling can locally modify the chemical properties of a molecule with a perturbed bond length.