  We have applied the graph theory to estimation of interactions between
electronic excitations in molecular crystals. Intramolecular and intermolecular
excitations have been considered. The intermolecular interactions contain
molecular integrals extended over two or three molecules, which we have called
dimer or trimer. The intermolecular interactions, such as \pi-\pi stacking, is
regarded as connections between neighboring molecules. To take up all dimers
and trimers in molecular crystal, we have treated the connections between
molecules as edges of a graph. The adjacency matrix of the graph and related
quantity are utilized to count all dimers and trimers systematically. The
absorption spectrum of TiOPc crystal in which we have defined three types of
intermolecular excitations is presented.
