Approximate DFT-based methods for generating diabatic states and calculating electronic couplings: models of two and more states

How the Donor/Acceptor Spin States Affect the Electronic Couplings in Molecular Charge-Transfer Processes?

The electronic coupling matrix element H_AB is an essential ingredient of most electron-transfer theories. H_AB depends on the overlap between donor and acceptor wave functions and is affected by the involved states’ spin. We classify the spin-state effects into three categories: orbital occupation, spin-dependent electron density, and density delocalization. The orbital occupancy reflects the diverse chemical nature and reactivity of the spin states of interest. The effect of spin-dependent density is related to a more compact electron density cloud at lower spin states due to decreased exchange interactions between electrons. Density delocalization is strongly connected with the covalency concept that increases the spatial extent of the diabatic state’s electron density in specific directions. We illustrate these effects with high-level ab initio calculations on model direct donor–acceptor systems relevant to metal oxide materials and biological electron transfer. Obtained results can be used to benchmark existing methods for H_AB calculations in complicated cases such as spin-crossover materials or antiferromagnetically coupled systems.

How To Extract Quantitative Information on Electronic Transitions from the Density Functional Theory "Black Box"

Electronic couplings and vertical excitation energies are crucial determinants of charge and excitation energy transfer rates in a broad variety of processes ranging from biological charge transfer to charge transport through inorganic materials, from molecular sensing to intracellular signaling. Density Functional Theory (DFT) is generally used to calculate these critical parameters, but the quality of the results is unpredictable because of the semiempirical nature of the available DFT approaches. This study identifies a small set of fundamental rules that enables accurate DFT computation of electronic couplings and vertical excitation energies in molecular complexes and materials. These rules are applied to predict efficient DFT approaches to coupling calculations. The result is an easy-to-use guide for reliable DFT descriptions of electronic transitions.