From simple electrostatics the core-core repulsion integral in eV is:

E_{N}(A,B) = 14.399 Z_{A}Z_{B}/R_{AB}

However, the electron-electron and electron-core integrals do not collapse to the form c/R_{AB} (R in Å) for distances beyond the van der Waal's radii. If the simple term given above is used, there would be a net repulsion between two neutral atoms or molecules. To correct for this the core-core repulsion is approximated by: