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.