Studies on the intrapair and interpair correlation of electrons of HX

The intrapair and interpair correlation energies of electrons of HX(X = Li - F, HBe+, HBe) have been studied by CISD(configuration interaction with si ngly and doubly excited configurations)method with 6-311 + G* basis set. It has been found that the electron intrapair and interpair correlation energ ies are very sensible to orbitals (except the inner orbitals). Furthermore, the changes of intrapair and interpair energies of valence electrons are v ery great in the different element - formed systems. Under the conditions o f the same theoretical method and the same basis sets, the number of electr ons will affect directly the values of interpair correlation energies of el ectrons. If a system has many electrons, the total electron interpair corre lation energy will dominate in the total electron correlation energy. This reminds 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.