The intrapair and interpair correlation energies of electrons of HX (X=Li-F, HBe^+, HBe) have been studied by CISD(configuration interaction with singly and doubly excited configuration)method with 6-311+G^* basis set. It has been found that the electron intrapair and interpair correlation energies are very sensible to orbitals (except the inner orbitals). furthermore, the changes of intrapair and interpair energies of valence electrons are very great in the different element-formed systems. Under the conditions of the same theoretical method and the same basis sets, the number of electrons will affect directly the vlues of interpair correlation energies of electrons. If a system has many electrons, the total electron interpair correlation energy will dominate in the total electron correlation energy. this remine\ds us that we should take the importance of interpair of electrons and the difference of orbitals into consideration while further correcting the caculated result of Post-HF.

Key words: LITHIUM HYDRIDE, HYDROGEN FLUORIDE