The transferable properties of pair correlation energies of the F-2 component are investigated in the alkali metal diflouride molecules MF2 (M = Li, Na, K). The F-2- correlation energy contributions in MS molecules are discussed and compared with the sum of pair correlation energies of the F-2 molecule and F-2(-) anion. A simple method is proposed to estimate the correlation energy for ionic crystal compounds. This method can be used for the prediction and estimation of the correlation energy For large ionic molecules requiring much less computation work.