Importance of dynamical electron correlation in diabatic couplings of electron-exchange processes

We demonstrate the importance of the dynamical electron correlation effect in diabatic couplings of electron-exchange processes in molecular aggregates. To perform a multireference perturbation theory with large active space of molecular aggregates, an efficient low-rank approximation is applied to the complete active space self-consistent field reference functions. It is known that kinetic rates of electron-exchange processes, such as singlet fission, triplet-triplet annihilation, and triplet exciton transfer, are not sufficiently explained by the direct term of the diabatic couplings but efficiently mediated by the low-lying charge transfer states if the two molecules are in close proximity. It is presented in this paper, however, that regardless of the distance of the molecules, the direct term is considerably underestimated by up to three orders of magnitude without the dynamical electron correlation, i.e., the diabatic states expressed in the active space are not adequate to quantitatively reproduce the electron-exchange processes.

Automated summary

We demonstrate the importance of dynamical electron correlation effect in diabatic electron-exchange processes in molecular aggregates. A multireference perturbation theory with a large active space is performed using an efficient low-rank approximation. It is found that the direct term of diabatic couplings is not sufficient to explain the kinetic rates of electron-exchange processes, such as singlet fission, triplet-triplet annihilation, and triplet exciton transfer, but these processes are efficiently mediated by low-lying charge transfer states if the two molecules are in close proximity. It is also observed that regardless of the distance between the molecules, the direct term is significantly underestimated without the dynamical electron correlation.