Conical Intersections Studied by the Configuration-Interaction-Corrected Tamm-Dancoff Method.

Conical intersections directly mediate the internal energy conversion in photoinduced processes in a wide range of chemical and biological systems. Because of the Brillouin theorem, many conventional electronic structure methods, including configuration interaction with single excitations from a Hartree-Fock reference and time-dependent density functional theory in either the linear response approximation (TDDFT) or Tamm-Dancoff approximation (DFT-TDA), have the wrong dimensionality for conical intersections between the ground state (S₀) and the first excited state (S₁) of the same multiplicity. This leads to unphysical state crossings. Here, we implement and assess the configuration-interaction-corrected Tamm-Dancoff approximation (CIC-TDA) that restores the correct dimensionality of conical intersections by including the coupling between the reference state and the intersecting excited state. We apply the CIC-TDA method to the S₁/S₀ conical intersections in ammonia (NH₃), ethylene (C₂H₄), bithiophene (C₈H₆S₂), azobenzene (C₁₂H₁₀N₂), and 11-cis retinal protonated Schiff base (PSB11) in vacuo. We show that this black-box approach can produce potential energy surfaces (PESs) of comparable accuracy to multireference wave function methods. The method validated here can allow cost-efficient explorations of photoinduced electronically nonadiabatic dynamics, especially for large molecules and complex systems.