The electronic structure of the QM tetramers was modeled with the rAM1/FOMO-CIS method. The method is configuration interaction singles (CIS) based on molecular orbitals (MO) obtained from a self-consistent field calculation with floating occupation (FO) numbers 47 using the Austin Model 1 (AM1) that was reparameterized (r) for azobenzene. For the CIS calculations a restricted active space of eight highest occupied and four lowest virtual orbitals was used. This active space includes orbitals originating from HOMO-1 (π), HOMO (n) and LUMO (π * ) of a monomer (see Figure ), and it was found to yield satisfactory absorption spectra (see the next section). [HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied (virtual) molecular orbital.] In total, 65 Slater determinants are used to construct electronic wave functions.