“…In anthracene there are two molecules per unit cell, and the appropriate linear combinations of the one-site excitons for the case where k is parallel to or perpendicular to the monoclinic axis are The term J~l,ml is the excitation transfer integral and is given by (10) (12) and the term K!l,m/l is the electron exchange term, which is given by The Davydov splitting, which is probably the main contact between theory and experiment, is given by the difference in energy between the two states M f + and M f -at k = 0, and by far the largest amount of activity in exciton theory in molecular crystals has been devoted to attempts to calculate this splitting.…”