In the past few decades, quantum chemistry calculations with density functional theory (DFT) have been widely used in studying PAH and its aggregates, including reproducing structural parameters, analyzing inter-molecular interactions, and predicting absorption/fluorescence spectroscopies. Alvarez-Ramírez and Ruiz-Morales systematic study on thousands of asphaltene-like PAH dimers showed that asphaltene dimer's optical gap falls within the experimental fluorescence energy range 1.9-3.1 eV if at least one of its monomer's optical gap is within this range. Later, time-dependent density functional theory (TDDFT) studies on PAH aggregates in combustion soot showed that emission energies of PAH dimers are proportional to their monomer's average HOMO-LUMO gap and usually do not exceed the fluorescence energy of the larger PAH. Extensive excited state potential energy surface (PES) scanning of dimers of benzene, naphthalene, pyrene, and perylene indicate that the intermolecular configurations with the largest number of overlapped sixmembered rings, namely the eclipsed configuration, exhibit the lowest fluorescence energy.