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State-averaged (SA) variants of the spin-restricted ensemble-referenced Kohn-Sham (REKS) method, SA-REKS and state-interaction (SI)-SA-REKS, implement ensemble density functional theory for variationally obtaining excitation energies of molecular systems. In this work, the currently existing version of the SA-REKS method, which included only one excited state into the ensemble averaging, is extended by adding more excited states to the averaged energy functional. A general strategy for extension of the REKS-type methods to larger ensembles of ground and excited states is outlined and implemented in extended versions of the SA-REKS and SI-SA-REKS methods. The newly developed methods are tested in the calculation of several excited states of ground-state multi-reference systems, such as dissociating hydrogen molecule, and excited states of donor-acceptor molecular systems. For hydrogen molecule, the new method correctly reproduces the distance dependence of the lowest excited state energies and describes an avoided crossing between the doubly excited and singly excited states. For bithiophene-perylenediimide stacked complex, the SI-SA-REKS method correctly describes crossing between the locally excited state and the charge transfer excited state and yields vertical excitation energies in good agreement with the ab initio wavefunction methods. [ABSTRACT FROM AUTHOR]

Electron transfer in the ground and excited states of a model donor-acceptor (D-A) system is investigated using the single-reference and multi-reference density functional theory (DFT) methods. To analyze the results of the calculations, a simple two-site multi-reference model was derived that predicts a stepwise electron transfer in the S0 state and a wave-like dependence of the S1 electron transfer on the external stimulus. The standard single-reference Kohn-Sham (KS) DFT approach and the time-dependent DFT (TDDFT) method failed to describe the correct dependence of the S0 and S1 electron transfer on the external electric field applied along the donor-acceptor system. The multi-reference DFT approach, the spin-restricted ensemble-referenced KS (REKS) method, was able to successfully reproduce the correct behavior of the S0 and S1 electron transfer on the applied field. The REKS method was benchmarked against experimentally measured gas phase charge transfer excitations in a series of organic donor-acceptor complexes and displayed its ability to describe this type of electronic transitions with a very high accuracy, mean absolute error of 0.05 eV with the use of the standard range separated density functionals. On the basis of the calculations undertaken in this work, it is suggested that the non-adiabatic coupling between the S0 and S1 states may interfere with the electron transfer in a weakly coupled donor-acceptor system. It is also suggested that the electronic excitation of a D+-A- system may play a dual role by assisting the further electron transfer at certain magnitudes of the applied electric field and causing the backward transfer at lower electric field strengths. [ABSTRACT FROM AUTHOR]