The electronic properties of the photosynthetic reaction center of photosystem I have been investigated on the basis of the X-ray structure of Synechococcus elongatus. The exciton coupling between nearest neighbors is calculated to be of a similar magnitude and about 330 cm(-1). To match the calculated to the experimental transient difference spectra (P-700(+) - P-700) and (A(0)(-) - A(0)) an angle of 40 +/- 10 degrees between the Q(y) transitions of the special pair chlorophylls was required. The effect of separating and translating the special pair chlorophylls was also investigated. The localizing effect on the exciton states of adding disorder to the intermolecular couplings was studied.