Quantum mechanical methods applied to excitation energy transfer: A comparative analysis on excitation energies and electronic couplings

We present a comparative study on the influence of the quantum mechanical QM method including basis set on the evaluation of transition energies, transition densities and dipoles, and excitation energy transfer EET electronic couplings for a series of chromophores and the corresponding pairs typically found in organic electro-optical devices and photosynthetic systems. On these systems we have applied five different QM levels of description of increasing accuracy ZINDO, CIS, TD-DFT, CASSCF, and SAC-CI. In addition, we have tested the effects of a surrounding environment either mimicking a solvent or a protein matrix on excitation energies, transition dipoles, and electronic couplings through the polarizable continuum model PCM description. Overall, the results obtained suggest that the choice of the QM level of theory affects the electronic couplings much less than it affects excitation energies. We conclude that reasonable estimates can be obtained using moderate basis sets and inexpensive methods such as configuration interaction of single excitations or time-dependent density functional theory when appropriately coupled to realistic solvation models such as PCM.