Triplet Excitation Energy Transfer with Constrained Density Functional Theory

We study the electronic coupling matrix element for triplet excitation energy-transfer processes with a number of different computational methods. For the first time, constrained density functional theory (CDFT) is applied to the problem of energy transfer, and results are compared with direct coupling calculations of broken symmetry and fragment densities, as well as the splitting method. A naive calculation of the electronic coupling using diabatic and adiabatic energy differences is shown to yield erroneous results due to the fractional spin error present in both Hartree-Fock and commonly used DFT exchange-correlation functionals. Some potential issues concerning the splitting method with triplet references within Hartree-Fock and DFT are discussed. We find :hat only methods that compute the matrix element directly (either from CDFT, broken symmetry, or fragment states) appear to be robust. Several illustrative examples are presented.