Berlin 2012 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 31: Organic semiconductors II

CPP 31.9: Talk

Thursday, March 29, 2012, 12:00–12:15, C 130

Non-empirical Determination of the Fraction of Exact Exchange in Density-functional Theory — •Viktor Atalla¹, Mina Yoon², and Matthias Scheffler¹ — ¹Fritz-Haber-Institut der MPG, Berlin, Germany — ²Oak Ridge National Laboratory, USA

Calculating the electronic structure of small molecules with density-functinoal theory (DFT) is challenging because conventional exchange-correlation (XC) functionals typically severely underestimates HOMO-LUMO gaps, which may result in a wrong description for charge transfer between weakly interacting molecules.

Here we consider the PBE0 [1] types of XC functionals using the fraction of exact exchange (α) as an adjustable parameter. Employing the G⁰W⁰ approach we identify the optimum α (α₀) for which the quasi particle correction to the Kohn-Sham HOMO vanishes. We tested this scheme for the G2 datasets of small molecules and find α₀ values in the range of 0.7−0.9. The generalized Kohn-Sham HOMO levels at the α₀ have a mean error of ∼ 0.3 eV with respect to the experimental vertical ionization potentials.

We apply the scheme for the prototypical donor-acceptor complex TTF/TCNQ and obtain α₀ ∼ 0.8, which gives an electronic level alignment consistent with the experimental value. It follows that this system rather exhibits intra-molecular electron-density rearrangement then charge transfer.

[1] C. Adamo, V. Barone, J. Chem. Phys. 110, 6158 (1999)