Dear Q-Chem users,
I am recently studying how to calculate excitation energy transfer using fragment excitation difference (FED) method. I have some questions about the details of this method:
(1) Does the definition of donor restricts that electrons must be excited from the donor regions? If so, how can only excitation calculation (for example, CIS) obtain the electronic coupling between donor and acceptor?
(2) What is the meaning of eignstates |m> |n> (or denoted |i> and |f>)? (i) From the perspective of avoided crossing, they are two states whose energies are split by the perturbation. Due to the mixing of states, it is difficult to assign |m> or |n> to either the state of reactant or product near avoided crossing;
(ii) On the other hand, according to [J. Phys. Chem. C, Vol. 112, No. 4, 2008] , |m> and |n> are defined as the reactant (initial) and product (final) state, respectively. Besides, when considering the exciton density difference, |m> and |n> seem to denote the state of reactant and product, respectively. And this also makes more sense in explaining the transition state |mn>.
(3) Why the difference of exciton density (donor minus acceptor) is used as the criteria rather than the summation? If the formed exciton is delocalized between donor and acceptor, should not we consider the sum of exciton density in donor and acceptor region?
Since I am trying to repeat the results of a published paper (J. Phys. Chem. Lett. 2018, 9, 6638−6643), which applied FED in other calculation software, it is important for me to understand FED method deeper. Therefore, I would appreciate it if someone could answer these, or one of these questions!