The excitonic basis is said to be strongly orthogonal if for any two site state products Φ and Θ Φ(x 1 , . . . , x N el )Θ(x 1 , . . . , x N el )dx 1 = 0 (4) holds for all possible values of x i , i = 2, . . . , N el , where N el is the total number of electrons in the system. As will be shown, the GFT expressions obtained under this assumption can be evaluated entirely from the site energies and site-specific 1-particle density matrices, and hence are much cheaper to evaluate than the exact matrix elements. Within SOA, the normalization constant for each antisymmetrized product reads