. Conical Intersections (CIs) and Interstate Crossings (ISCs) are the specific manifolds on the Potential Energy Surfaces (PESs) of two different electronic states where, at the same geometry, the energies of these states are strictly equal each other and thus two states are exactly degenerate. The difference between CIs and ISCs is that ISCs are always formed by the states of different spatial or spin symmetry. As a result these states belong to the different blocks of the Hamiltonian matrix expressed in a

25. Firefly documentation - state-specific gradients for state-averaged MCSCF and