Ab initio mo calculations on the chlorophyll–water system and estimation of the structure of the special pair

Ab initio LCAO SCF MO calculations were carried out on the chlorophyll–water system, which is considered as a moiety of the so‐called special pair in the reaction center of photosynthesis. The ionization potential and electron affinity of the system were 6.16 and 0.30 eV from the ΔSCF calculations. By using the atom–atom potential method for describing intermolecular interactions, we estimate the structure of the pair to be very close to that proposed by Shipman and others. The two major contributions to interaction energy are due to hydrogen bonds with water molecules and dispersion between the two chlorophyll planes, respectively.