Electron Correlation

Abstract

Hartree-Fock method quantitatively reproduces electronic structure. However, electron–electron interaction, which is called electron correlation effect, is theoretically treated in an average manner. For example, Coulomb hole cannot be quantitatively represented, though Fermi hole can be represented. To incorporate electron correlation effect accurately, several calculation methods beyond Hartree-Fock such as configuration interaction (CI), coupled cluster (CC), density functional theory (DFT) have been developed. In CI and CI-based CC methods, it is assumed that the exact wave-function is represented by the combinations of the wave-functions of several excited electron configurations. Though CC method succeeded in reproducing electronic structure of small molecules, CI and CI-based CC methods essentially contain the scientific contradiction that the summation of several Hartree-Fock equations is away from universal quantum concept. DFT has the different concept to incorporate electron correlation effect. The electron correlation effect is directly considered to represent the correct exchange-correlation energy. Though universal exchange-correlation functional has not been developed, DFT predicts correct electronic state in transition metal compounds.