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Abstract

Recent studies of hybrid layered interfacial systems have revealed remarkable electronic, visible light response and (photo)catalytic properties. Such properties appear often to be intimately related to intrinsic charge transfer properties of the hybrid interfacial systems in their ground electronic states. Hence, a sound fundamental understanding of charge transfer properties of complexes and interfaces is an important objective. In this work, we make a detailed exploration of the charge transfer properties of a canonical electron donor-acceptor pair: the Tetrathiafulvalene (TTF)-Tetracyanoquinodimethane (TCNQ) dimer. Our study shows that the simple frontier-orbital based picture of electron transfer for this system as derived from conventional DFT calculations is both qualitatively and quantitatively erroneous, in the sense that it rationalizes the experimentally known electronic properties for the wrong reasons. A recently developed DFT approach that alleviates self-interaction errors (Physical Review B 88, 165122) is utilized to illustrate that charge transfer effects in the TTF-TCNQ dimer are not only much smaller than predicted by conventional (semi)local functionals such as Perdew-Burke-Ernzerhof (PBE) and local density approximation (LDA), but in addition are highly asymmetric – depending sensitively on relative orientation of the two molecules.