Search

The operator of electrostatic interactions was applied to determine the coefficient for the 30 molecular states that arise from the interaction of two Sc (D); the Slater-Kirkwood formula was used to approximate . The potential energy curves of some states were determined with the complete-active-space self-consistent-field and multireference configuration interaction methods and then extrapolated to the complete basis-set limit. Comparable results of , , , , and were approximated by least-squares fits of the curves for some states. An analysis of the role of static and dynamic correlation energy in this procedure is assessed. In the long-range region of the curves of singlet states, the static correlation energy accounts for dispersion energy, which causes errors when the dynamic correlation is fitted to determine . [ABSTRACT FROM AUTHOR]

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]