Kawai et al. reported the crystal structure of a dispirobis(10-methylacridan) derivative that contains a C-C single bond over 1.77 angstrom long. In this Letter, quantum chemical calculations of the compound and its derivatives for the isolated and the crystal systems were performed. These calculations demonstrated that the intramolecular and intermolecular dispersion interactions, as well as the crystal packing effect, are important to realize such an ultralong C-C bond length.