From simple electrostatics the core-core repulsion integral in eV is:
EN(A,B) = 14.399 ZAZB/RAB
However, the electron-electron and electron-core integrals do not collapse to the form c/RAB (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:
EN(A,B)=ZAZB<AA|BB>.
The MNDO approximation to the screening effect is :
EN(A,B) = ZAZB<sAsA|sBsB>(1 +e-αARAB +e-αBRAB).
Again, O-H and N-H interactions are treated differently. For these interactions, use
EN(A,B) = ZAZB<sAsA|sBsB>(1 +RABe-αBRAB +e-αHRAB).