Calculation of coherences in Förster and modified Redfield theories of excitation energy transfer.

Förster and modified Redfield theories play one of the central roles in the description of excitation energy transfer in molecular systems. However, in the present state, these theories describe only the dynamics of populations of local electronic excitations or delocalized exciton eigenstates, respectively, i.e., the diagonal elements of the density matrix in the corresponding representation. They do not give prescription for propagating the off-diagonal elements of the density matrix (coherences). This is commonly accepted as a limitation of these theories. Here, we derive formulas for the dynamics of the coherences in the framework of Förster and modified Redfield theories and, thus, remove this limitation. For the dimer case, these formulas provide excellent correspondence with numerically exact calculations according to the hierarchical equations of motion.