where blocks like H 0T , H SQ , etc., are zero according to the excitonic analogue of the Slater-Condon rules. The detailed structure of an exemplary Hamiltonian matrix-as well as which ECI formulas are used to calculate which matrix elements-is illustrated in Figure . In excitonic terms, the block H 0S contains the GS-LE couplings, which are neglected in the FEM, but not a priori not in ECI. Note that, unlike in the FEM, we also in principle consider the couplings between two local excitations on the same site (those are part of H SS ). Both types of couplings are covered by the modified expression ( ), although we note that this expression is more general and covers the couplings between any ESDs differing in one site state, regardless of their excitonic excitation ranks (GS, LE, DLE, Figure : (a) Schematic representation of the excitonic basis of ECIS, ECISD, and ECISDT bases on an example of a tri-chromophoric system with three site states (ground and two excited) supplied on each fragment. The chromophores are represented by hexagon, pentagon, and square, while the site states are denoted by colour (white: ground site state, purple and green: two excited site states).