The vectors g IJ and h IJ span a two-dimensional branching plane at the CX, and the degeneracy between the two intersecting states is preserved through first order (δR), provided that δR is restricted to the space orthogonal to the branching plane, i.e., to the seam space. If the CX involves the reference state (meaning the HF ground state, for conventional CIS), then the derivation presented in Appendix A shows that the nonadiabatic coupling vector can be written