Off-diagonal terms One problem with this approach is the equal division of the off-diagonal terms between the two basis functions. This leads to charge separations in molecules that are exaggerated. In a modified Mulliken population analysis, this problem can be reduced by dividing the overlap populations \mathbf {P_{\mu\nu}} between the corresponding orbital populations \mathbf {P_{\mu\mu}} and \mathbf {P_{\nu\nu}} in the ratio between the latter. This choice, although still arbitrary, relates the partitioning in some way to the
electronegativity difference between the corresponding atoms.
Ill definition Another problem is the Mulliken charges are explicitly sensitive to the basis set choice. In principle, a complete basis set for a molecule can be spanned by placing a large set of functions on a single atom. In the Mulliken scheme, all the electrons would then be assigned to this atom. The method thus has no complete basis set limit, as the exact value depends on the way the limit is approached. This also means that the charges are ill defined, as there is no exact answer. As a result, the basis set convergence of the charges does not exist, and different basis set families may yield drastically different results. These problems can be addressed by modern methods for computing net atomic charges, such as density derived electrostatic and chemical (DDEC) analysis, intrinsic atomic orbitals, electrostatic potential analysis, and natural population analysis. == See also ==