The most well-known exciton model applicable to molecular multichromophoric systems is the Frenkel exciton model (FEM). It employs an excitonic basis that consists only of the product of the site ground states (the so-called GS product) and all the products with exactly one excited site state, referred to as the local excitation (LE) products. The expressions for the matrix elements in FEM, although not historically obtained like that, actually correspond to the GFT expressions with some additional approximations (as will be recapitulated below).