The accurate description of the excited electronic states of molecular systems is of central importance for the simulations of electronic spectra and photoinduced molecular dynamics. Nowadays, a large array of electronic-structure methods for such calculations is available, 1 ranging from single-2 to multi-reference 3 ones. However, all of those methods scale unfavorably (i.e., polynomially with large exponent) with the system size and therefore in practice cannot be employed to treat large systems of interest. Of particular relevance are multichromophoric systems-systems having low-lying excited states localized on different moieties (typically called chromophores, fragments, or sites, which we use interchangeably in this article). Such systems are ubiquitously found in nature (e.g., light-harvesting complexes in chloroplasts or DNA ) and have also gained significant attention in the field of material design (e.g., photovoltaics or organic semiconductors ). Hence, the understanding of their photophysical and photochemical properties is one of the highest priorities in modern spectroscopy. Unfortunately, if the spectra of many different chromophores are overlapping, multichromophoric systems can acquire a very large density of states at low energies, which makes their electronic structure very challenging to describe. For the same reason, the ultrafast excitation energy transfer (EET) that takes place in such systems after a local photoexcitation is very hard to simulate with excited-state molecular dynamics.