FDE-ET introduces an additional approximation to the generalized eigenvalue problem described above enabling the use of an analytical expression for electronic couplings. This can be achieved by i) assuming that S 11 = S 22 , ii) dividing the off-diagonal element S 12 by √ S 11 S 22 (to obtain S 12 ), and iii) omitting the term S nn on the right-hand side of Eq. ( ) for the case of diagonal elements such that