An alternative to traditional electronic structure methods for the treatment of multichromophoric systems are so-called exciton models. The idea behind exciton models is to compute individual chromophores separately (obtaining so-called site states) in a first step, and then to construct the full-system model Hamiltonian matrix in the basis of the antisymmetrized products of the fragments' states (a so-called excitonic basis). After the model Hamiltonian matrix is obtained, the full-system states are obtained by its diagonalization. The approximations applied in the exciton models are related either to the truncation of the basis of the antisymmetrized products or to approximations used in evaluating the matrix elements. Worth mentioning, there are fragment-based methods that employ a similar philosophy but evaluate the full-system Hamiltonian matrix in the basis of antisymmetrized products (in principle) exactly, e.g., the Localized Active Space State Interaction (LASSI) or Active Space Decomposition (ASD) methods. Such "exact matrix element" frameworks can also be implemented by other general (not necessarily fragment-based) approaches, such as the Generalized Active Space (GAS) methods, and allow studying multi-fragment systems with very strongly correlated fragments. However, if the chromophores are not particularly close to each other, one can assume that the antisymmetrized products are strongly orthogonal. This is called the strong orthogonality assumption (SOA). The SOA leads to much simpler and computationally less demanding expressions for the Hamiltonian matrix elements, and hence many exciton models rely on it. The corresponding expressions were obtained by McWeeny in his group function theory (GFT), and were used to study intermolecular interactions.