Intermolecular pi-pi stacking interactions of the bacteriochlorophyll dimer in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides were analyzed by the second order Moller-Plesset perturbation method using the modified 6-31G*(0.25) basis set with diffuse d-polarization by Hobza and co-workers. MP2/6-31G*(0.25) calculations yield an intermolecular interaction energy of -21.50 kcal/mol for the bacteriochlorophyll dimer. Thus, the attractive nature of the pi-pi stacking interaction of the bacteriochlorophyll dimer in the photosynthetic reaction center from Rhodobacter sphaeroides is, for the first time, firmly established.