Study on Pair Correlation Energies of Isoelectronic Systems F^-,HF and H_2F^+

The intrapair and interpair correlation energies of F -, HF and H 2F +systems are calculated and analyzed using MP2 OPT2 method of MELD program with cc PV5Z * basis set. From the analysis of pair correlation energies of these isoelectronic systems, it is found that the 1s F 2 pair correlation energy is transferable in these three isoelectronic systems. According to the definition of pair correlation contribution of one electron pair to a system, the pair correlation contribution values of these three systems are calculated. The correlation contribution values of inner electron pairs and H—F bonding electron pair in HF molecule with those in H 2F +system are compared. The results indicate that the bonding effect of a molecule is one of the important factors to influence electron correlation energy of the system. The comparison of correlation energy contributions including triple and quadruple excitations with those only including singles and doubles calculated with 6 311++G(d) basis set shows that the higher excitation correlation energy contribution gives more than 2% of the total correlation energy for these systems.