With a singlet exciton split into two triplet excitons,
singlet fission has a great potential to increase the
efficiency of solar cells. The Fragment Spin Difference
(FSD) scheme was generalized to calculate the singlet
fission coupling. Without manually including the CT
components, the largest coupling strength obtained was
14.8 meV for two pentacenes in a crystal structure, or
33.7 meV for a transition-state structure, which
yielded singlet fission lifetime of 239 or 37 fs,
generally consistent to experimental result (80 fs). We
found that the charge on one fragment in the S1
diabatic state correlates well with FSD coupling,
indicating the importance of the CT component. The FSD
approach is a useful first-principle method for singlet
fission coupling, without the need to include the CT
component explicitly. In this talk I'll also discuss on various aspects
explored behind the scene.