Exchange and dynamic correlation.

In Density Functional Theory (DFT) the exchange-correlation functional is often written as a sum of exchange and correlation parts. We give reasons for thinking that OPTX is a very good exchange functional. We ask the question 'what is the equivalent quantum chemistry calculation which gives energies close to those obtained from Kohn-Sham calculations with OPTX?' OPTX was determined by refining its parameters so that atomic Hartree-Fock energies (of H-Ar) were reproduced. For H2 we find that OPTX values are very close to those obtained with the Heitler-London wavefunction with the 1s atomic orbitals on the protons having an energy-optimised exponent as first calculated by Wang. This holds at equilibrium and stretched geometries. The consequences of this investigation on the definition of dynamic correlation are discussed. OPTX wavefunctions for other molecules are suggested. [ABSTRACT FROM AUTHOR]