Charge-Transfer Complexes:  Stringent Tests for Widely Used Density Functionals

Density functional calculations are reported for charge-transfer complexes (CT), also called electron donor−acceptor systems, formed from ethylene or ammonia interacting with a halogen molecule (C 2 H 4 ···X 2, NH 3 ···X 2, X = F, Cl, Br, and I). In all cases, the local density approximation provides a strong overestimation of the intermolecular interaction. The generalized gradient approximation moves the results in the right direction but, in general, not nearly far enough; large errors remain. We attribute the problem to the too rapid asymptotic decay of the exchange−correlation potential associated with the imperfect cancellation of the self-interaction. This breakdown of the potential is reflected in a set of incorrect eigenvalues (orbital electronegativities) that play a crucial role in governing the charge transfer and, hence, the interaction energy. The inclusion of some Hartree−Fock exchange using hybrid methods provides a large improvement, and the parameters related to the intermolecular interaction for the so-called half-and-half potential are in very good agreement with those obtained through second-order Møller−Plesset calculations and with available experimental data. However, the more widely used three-parameter, B3LYP, functional does not perform well; the hybrid methods are not a panacea.