Core-core repulsion integrals

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:


MNDO modification to the core-core term

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).