Systematic exploration of minimum energy conical intersection structures near the Franck-Condon region.

Locating accessible conical intersections (CIs), especially minimum energy CI (MECI) structures, near the Franck-Condon (FC) region is one of the most important tasks in theoretical analyses of photoreactions. Many MECIs may exist around a FC point in molecules with many vibrational degrees of freedom. Usually, MECIs are optimized one by one starting from arbitrary chosen initial structures. In order to eliminate the arbitrariness, we have developed automated MECI search methods. In this paper, a new approach is described. It combines the seam model function approach with the recently proposed single-component artificial force induced reaction method. Starting from a FC point, the present method finds MECIs systematically. It requires neither a Hessian nor a derivative coupling vector. In an example of the automated search, the spin-flip TDDFT was employed as an efficient electronic structure calculation method, which, together with an automated algorithm to recognize proper electronic states, allowed for evaluation of energy and gradient in a black-box fashion. The present approach was tested with trans- and cis-1,3-butadiene, thymine, and coumarin molecules. The usefulness of the present approach was demonstrated by comparing obtained MECIs with those in the literature. It is hoped that the present technique will be useful in exploration of unknown photoreaction pathways.