The equivalence condition of the correlation energy definition in Hartree-Fock method (HF) and in Density Functional Theory (DFT) is analyzed. It is shown that when the reference electron density is far from the actual density of the system concerned, two definitions don't give the same values and the DFT correlation energy is larger than the HF one in magnitude. Meanwhile, the correlation energy calculated by the hybrid approach of DFT and HF is smaller than the HF value in magnitude, the difference being explained by the stationary correlation energy. It is also verified that the difference can be estimated by limited CI method with appropriate configurations. The potential curves of several diatomic molecules H2 (X^1∑g^+), HF (X^1∑^+), N2 (^1∑g^+) are computed, all are better than those by CISD or MP4 method. Furthermore, the calculated dissociation energy of H2 is 0.17a. u., in agreement with the experimental value, 0.1747 a. u..

Key words: CONFIGURATION