The Role of Exact-Exchange in the Theoretical Description of Organic-Metal Interfaces

We investigate the ability of different density functional methods to describe the electronic properties of isolated gold clusters, self-assembled monolayers (SAM) of oligophenylthiols (including the depolarization effect), and the biphenylthiol/gold interface. To elucidate the role of the exchange interaction, we consider a hierarchy of functionals including conventional (e.g., within the gradient corrected approximation), hybrid, and effective exact-exchange functionals, namely the Localized Hartree-Fock (LHF) method, which is free from the self-interaction-error (SIE). We find that conventional exchange-correlation functionals cannot well describe the energy-level alignment at the metal/organic interface and predict a negligible metal-molecule charge-transfer. In addition, an overestimation of dipole moments and polarization effects are obtained in oligophenylthiols, leading to a wrong description of the SAM depolarization effect. Both limitations are mostly overcome if exact-exchange contributions are taken into account either using an hybrid functional or the LHF method. In particular, an accurate description of the metal/organic interface is only achieved using SIE free methods. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110: 2162-2171, 2010