where N F s F is the number of site states of spin s F provided on site F . As can be seen, N full ECI grows very quickly and can easily become too large to be handled. In analogy with regular CI, the GS product in ECI plays the role of the aufbau configuration, the LE products play the role of excitonic single excitations, and the MLE products play the role of excitonic double (DLE), triple (TLE), and higher excitations. Thus, like in CI, we can truncate the full ECI basis according to the excitation rank of the ESDs, obtaining the methods ECIS, ECISD, and so on. Full ECI is then ECISD. . . M , while the basis used in the FEM corresponds to the ECIS expansion. The truncation of the ECI basis is schematically illustrated in Figure . Herein, we stress that the full ECI basis is still not complete, as it lacks the CT products, which would not have any analogy with the SDs.