The charge attributed to an atom A within a molecule defined as ζ = Z A- q A, where ZA is the atomic number of A and qA is the electron density assigned to A. The method of calculation of qA depends on the choice of the scheme of partitioning electron density. In the framework of the Mulliken population analysis qA is associated with the so-called gross atomic population: qA = ∑qµ, where qµ is a gross population for an orbital µ in the basis set employed defined according to \[q_{\unicode[Times]{x3BC}} = P_{\unicode[Times]{x3BC}\unicode[Times]{x3BC}}+\sum _{\begin{array}{c} \nu \neq \unicode[Times]{x3BC} \end{array}}P_{\unicode[Times]{x3BC}\nu}\ S_{\unicode[Times]{x3BC}\nu}\] where Pµν and Sµν are the elements of density matrix and overlap matrix, respectively (see overlap integral). In the Hückel molecular orbital theory (where Sµν = δµν), qµ = nµ.Pµµ, where nµ is the number of electrons in the MO µ.