Estimation of Electronic Coupling for Photoinduced Charge Separation and Charge Recombination Using the Fragment Charge Difference Method

Photoinduced electron transfer reactions play an important role in chemistry, biochemistry, and material sciences. Electronic coupling of donor and acceptor is a key parameter that controls the rate of charge separation and charge recombination processes. The fragment charge difference (FCD) method is extended to calculate the electronic couplings and diabatic energies for the photoinduced reactions. It is shown that FCD provides consistent values of the ET parameters for any 3-state model system. We compare the matrix elements obtained within the 2- and 3-state treatment for different situations and suggest how to check adiabatic states included in the diabatization procedure. Two examples demonstrate the use of the FCD method in combination with MS-CASPT2 calculations to derive the ET parameters.