Year: 2026
Type: preprint
Abstract: Exploring the potential energy surfaces of molecules in optical cavities requires analytical derivatives. We present analytical nuclear gradients and nonadiabatic derivative couplings for state-averaged quantum-electrodynamics complete-active-space self-consistent field theory (SA-QED-CASSCF) forthe coherent-state Pauli–Fierz Hamiltonian, including the coupledperturbed response of the QED-Hartree–Fock reference and the photon contribution to the coupling that arises because the coherent-state displacement depends on the geometry. The derivatives agree with finite differences to within 10−6 atomic units (5×10−6 for the state of smallest weight in an unequal average) for equal, unequal and zero-weight state averaging, and with independent codes without the cavity. With them we locate a polariton-induced conical intersection in LiH, where the cavity makes the molecular orientation a second coordinate and a reflection combined with photon parity makes the crossing symmetry-allowed. In ethylene, the S0/S1 conical intersection persists under coupling and keeps its twisted–pyramidalized geometry, and it becomes more accessible from the Franck–Condon point, because the dipole self-energy raises the diffuse π → 3s state more than the crossing pair. Capturing the partial return of this penalty through the bilinear coupling requires the polarization-axis partner orbital in the active space, a rule for polaritonic CASSCF in general.
Source: ChemRxiv
Authors: Nam Vu, Jonathan J. Foley
Institution: University of North Carolina at Charlotte
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220725320