As a side remark, we note that, if converged EHF charges are used as the APCs in an ECI calculation, the block H 0S = H † S0 is expected to be small, due to the excitonic analogue of Brillouin's theorem (see Chapter 14.3 in Ref. 15). If some other choice of APCs is made, the subsequent ECI calcuation is analogous to CI with some general (non-canonical) orbitals, so Brillouin's theorem does not hold and the H 0S block does not necessarily contain small values. Hence, ECI with non-EHF APCs not only employs a presumably less favorable variational basis, but it is also expected to be less size extensive, which can affect its performance.