The charge attributed to an atom \(A\) within a molecule defined as \(\zeta =Z_{\rm{A}}- q_{\rm{A}}\), where \(Z_{\rm{A}}\) is the atomic number of \(A\) and \(q_{\rm{A}}\) is the electron density assigned to \(A\). The method of calculation of \(q_{\rm{A}}\) depends on the choice of the scheme of partitioning electron density. In the framework of the Mulliken population analysis \(q_{\rm{A}}\) is associated with the so-called gross atomic population: \(q_{\rm{A}}=\sum q_{\unicode[Times]{x3BC}}\), where \(q_{\unicode[Times]{x3BC}}\) is a gross population for an orbital \(\unicode[Times]{x3BC}\) 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 \(P_{\unicode[Times]{x3BC}\unicode[Times]{x3BD} }\) and \(S_{\unicode[Times]{x3BC}\unicode[Times]{x3BD}}\) are the elements of density matrix and overlap matrix, respectively (see overlap integral). In the Hückel molecular orbital theory (where \(S_{\unicode[Times]{x3BC}\nu} = \delta _{\unicode[Times]{x3BC}\nu}\)), \(q_{\unicode[Times]{x3BC}} = n_{\unicode[Times]{x3BC}}\ P_{\unicode[Times]{x3BC}\unicode[Times]{x3BC}}\), where \(n_{\unicode[Times]{x3BC}}\) is the number of electrons in the \(\rm{MO}\,\unicode[Times]{x3BC}\).