Density-functional study of small molecules within the Krieger-Li-Iafrate approximation

We report density-functional studies of several small molecules (H-2, N-2, CO, H2O, and CH4) within the Krieger-Li-Iafrate (KLI) approximation to the exact Kohn-Sham local exchange potential, using a three-dimensional real-sp ace finite-difference pseudopotential method. It is found that the exchange -only KLI approximation leads to markedly improved eigenvalue spectra compa red to those obtained within the standard local-density approximation (LDA) , the generalized gradient approximation (GGA), and the Hartree-Fock (HF) m ethod. For structural properties, exchange-only KLI approximation results a re close to the corresponding HF values. We find that the addition of LDA o r GGA correlation energy functionals does not lead to systematic improvemen ts. [S1050-2947(99)08111-1].