Based on Kasha's rule, the electronic absorption spectra of molecular dimer should be obviously different from that of monomer due to the couplings between two monomers. It is thus favorable to directly calculate the excitation energies and the oscillator strengths (or pure electronic absorption spectra) of dimer to qualitatively understand the intermolecular interaction from the aggregation effects. Fig. displays the calculated results with a Gaussian broadening in the low-energy regime and the pure electronic absorption spectra of the corresponding monomers are shown for comparison. The DPND6 dimer exhibits two absorption peaks with quite different absorption strength, which are located at 2.45 eV and 2.59 eV and are blue-shifted compared with that of DPND6 monomer (at 2.41 eV). The DPND dimer also exhibits two absorption peaks with nearly identical strength, which are located at 2.34 eV and 2.83 eV, and appear on both sides of the monomer's peak (at 2.48 eV). These spectral properties of dimers cannot be straightforwardly explained by the traditional Kasha model, which states that the absorption peak of dimer should be blue-(red-) shifted for H-type (J-type) aggregates compared with the absorption peak of monomer, thus the more complex intermolecular interactions should be taken into account.