The vertical excitation energies for both the subunits and dimers were calculated using the density functional theory-based multireference configuration interaction DFT/MRCI method. The one-particle basis was calculated using the Becke "half-and-half" Lee-Yang-Par BHLYP hybrid density functional and the cc-pVDZ basis set, as implemented in the TURBOMOLE software package. In the case of the heterodimer, the damped Becke-Johnson variant of the geometrydependent 3-parameter DFT-D3(BJ) correction was used. The DFT/MRCI reference space was generated iteratively by including all electron configurations with expansion coefficients greater than 10 -3 in numerous probe calculations, using 10 electrons across 10 orbitals, allowing only a maximum of two-electron excitations. Probe runs were calculated by discarding configurations with energy less than the highest reference energy; starting with a threshold of 0.6Eh, then 0.8