Density-functional Theory and Beyond for Donor-Acceptor Complexes: The Example of TTF/TCNQ

We study the performance of density-functional theory (DFT) with various exchange-correlation (XC) functionals in describing electronic and structural properties of the prototypical donor/acceptor complex TTF/TCNQ. We find that the binding energetics and the amount of electron transfer between TTF and TCNQ depends strongly on the functional. In particular, all semilocal functionals give rise to significant, aritificial electron transfer due to a wrong ordering of Kohn-Sham (KS) levels. We consider the HSE [1] ``family'' of XC functionals using the fraction of exact exchange ($\alpha$) as adjustable parameter. The optimum XC functional is then identified as that for which the $G_0W_0$ quasiparticle correction to the energy gap of the KS LUMO of the acceptor and the HOMO of the donor is minimized. We obtain $\alpha \sim 0.8$ which gives an electronic level alignment that is consistent with experiment and free from spurious asymptotic charge transfer. We conclude that the proposed scheme improves the KS spectrum, and that the investigated TTF-TCNQ dimer exhibits intra-molecular electron-density rearrangement rather than electron transfer. \\[4pt] [1] A.V. Krukau, et al., J. Chem. Phys. {\bf 125}, 224106 (2006)