Finding Excited-State Minimum Energy Crossing Points on a Budget: Non-Self-Consistent Tight-Binding Methods

The automated exploration and identification of minimum energy conical intersections (MECIs) is a valuable computational strategy for the study of photochemical processes. Due to the immense computational effort involved in calculating non-adiabatic derivative coupling vectors, simplifications have been introduced focusing instead on minimum energy crossing points (MECPs), where promising attempts were made with semiempirical quantum mechanical methods. A simplified treatment for describing crossing points between almost arbitrary diabatic states based on a non-self-consistent extended tight-binding method, GFN0-xTB, is presented. Involving only a single diagonalization of the Hamiltonian, the method can provide energies and gradients for multiple electronic states, which can be used in a derivative coupling-vector-free scheme to calculate MECPs. By comparison with high-lying MECIs of benchmark systems, it is demonstrated that the identified geometries are good starting points for further MECI refinement with ab initio methods.

Automated summary

A simplified method based on the non-self-consistent extended tight-binding method GFN0-xTB is presented for describing crossing points between almost arbitrary diabatic states. The method involves only a single diagonalization of the Hamiltonian and can provide energies and gradients for multiple electronic states, which can be used to calculate minimum energy crossing points. The identified geometries are shown to be good starting points for further optimization of minimum energy conical intersections with ab initio methods, based on comparison with high-lying MECIs of benchmark systems.