Formulation and Implementation of the Spin-Restricted Ensemble-Referenced Kohn-Sham Method in the Context of the...

The spin-restricted ensemble-referenced Kohn-Sham (REKS) method and its state-interaction state-averaged variant (SI-SA-REKS, or SSR) provide computational platform for seamless inclusion of multireference effects into the density functional calculations. The SSR method enables an accurate calculation of the vertical excitation energies for the molecules with multireference ground states and describes conical intersections between the ground and excited states with the accuracy matching the most sophisticated ab initio multireference wave function methods. In this work, the SSR method is formulated and implemented in the context of the long-range corrected density functional tight binding (LC-DFTB) approach. The new LC-DFTB/SSR method enables calculation of the excited electronic states and the S₁/S₀ conical intersections of very large molecules. The LC-DFTB/SSR method is benchmarked against vertical excitation energies and conical intersection energies and geometries of several organic molecules with π/π* and n/π* transitions. It is demonstrated that the LC-DFTB/SSR method describes these molecules with reasonable accuracy, which can be considerably improved by a slight modification of the LC-DFTB spin polarization parameters.