A Merrifield-type Hamiltonian describing the coupling between the Frenkel and charge-transfer (CT) excitations is constructed for a finite cubic cluster of C-60 molecules. The model explicitly takes into account the off-diagonal disorder due to a random distribution of the CT integrals, resulting from the orientational disorder in the high-temperature fullerene phase. The Hamiltonian is numerically diagonalized; its eigenvalues and eigenvectors are used to calculate the electroabsorption signal. The spectrum is compared with that obtained for the previously proposed ad hoc model rooted in the mean-field approximation, where the crystal is assumed to have full translational symmetry and the disorder is included only implicitly by introducing effective matrix elements averaged over molecular orientations. The results confirm the validity of the mean-field approach. It is found to be very successful in reproducing the spectrum of the disordered crystal to a good approximation; the new results also justify a minor modification of the effective CT integrals, which still improves the agreement of the calculated spectrum with experiment. (C) 2003 American Institute of Physics.