The definition of correlation energy in Electron Gas Approximation (EGA) is not equivalent to that in Hartree-Fock framework. By comparing the physical model of the infinite electron gas with that of the proposed finite electron gas, which comprises finite number of electrons, the EGA correlation energy has been divided into three parts: self-correlation within one electron, spin-parallel correlation and Hartree-Fock correlation. The dependence of the contribution from each component on the electron number (N) has been described. The method of hybrid of Hartree-Fock and Density Functional Theory, generated from the above analysis, can be used to estimate the correlation energy of atoms and molecules without employing any empirical parameter. The average error of the calculation 4.2% is lower than those by conventional post Hartree-Fock methods, such as CI-SD and MP4.

Key words: QUANTUM CHEMISTRY, HARTREE-FOCK, ELECTRON GAS, ELECTRON CORRELATION