Conical Intersections in Organic Molecules: Benchmarking Mixed-Reference Spin-Flip Time-Dependent DFT (MRSF-TD-DFT) vs. Spin-Flip TD-DFT.

Abstract

The mixed-reference spin-flip time-dependent density functional theory (MRSF- TD-DFT) method eliminates the erroneous spin-contamination of the SF-TD-DFT method- ology while retaining conceptual and practical simplicity of the latter. The availabil- ity of the analytic gradient of the energy of the MRSF-TD-DFT response states en- ables automatic geometry optimization of the targeted states. Here, we apply the new method to optimize the geometry of several S1/S0 conical intersections occur- ring in typical organic molecules. We demonstrate that MRSF-TD-DFT is capable of producing the correct double cone topology of the intersections and to describe the geometry of the lowest energy conical intersections and their relative energy with an accuracy matching the best multi-reference wavefunction ab initio methods. In this regard, MRSF-TD-DFT differs from many popular single-reference methods, such as, e.g., the linear response TD-DFT method, which fail to produce the correct topology of the intersections. As the new methodology completely eliminates the ambiguity with the identification of the response states as proper singlets or triplets, which is plagu- ing the SF-TD-DFT calculations, it can be used for automatic geometry optimization and molecular dynamics simulations not requiring constant human intervention.