Core-core repulsion integrals

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

E_N(A,B) = 14.399 Z_AZ_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:

E_N(A,B)=Z_AZ_B<AA|BB>.

MNDO modification to the core-core term

The MNDO approximation to the screening effect is :

E_N(A,B) = Z_AZ_B<s_As_A|s_Bs_B>(1  +e^-α_A^R_AB  +e^-α_B^R_AB).

Again, O-H and N-H interactions are treated differently. For these interactions, use

E_N(A,B) = Z_AZ_B<s_As_A|s_Bs_B>(1  +R_ABe^-α_B^R_AB  +e^-α_H^R_AB).