Determining and extending the domain of exchange and correlation functionals

The current capabilities of density functional theory reach their limits somewhere in the region of weak inter- and intramolecular interactions. Hydrogen bonds can be treated, but not perfectly. The few results that have been obtained for dispersion-dominated van der Waals interactions are erratic. The generalized gradient approximation fails for charge-transfer complexes; hybrid functionals that incorporate a component of the Hartree–Fock exchange are promising, but the results depend strongly on the variant used. We recently used the adiabatic connection and coordinate scaling to develop correlation functionals based on a spin-polarized pair correlation function of the Colle–Salvetti type. A résumé of the current status of the functionals is given, including inter alia available validation results for various molecules and reactions, the water dimer, and charge-transfer complexes. It is shown that nonlocal schemes involving the kinetic energy density and, hence, the Laplacian of the electron density lead to an overall improvement of the calculated binding energies and equilibrium geometries, especially in cases when the current GGA schemes meet difficulties. © 1995 John Wiley & Sons, Inc.