The charge attributed to an atom $A$ within a molecule defined as $\zeta =Z_{\text{A}}-q_{\text{A}}$, where $Z_{\text{A}}$ is the atomic number of $A$ and $q_{\text{A}}$ is the electron density assigned to $A$. The method of calculation of $q_{\text{A}}$ depends on the choice of the scheme of partitioning electron density. In the framework of the Mulliken population analysis $q_{\text{A}}$ is associated with the so-called gross atomic population: $q_{\text{A}}=\sum q_{\text{μ}}$, where $q_{\text{μ}}$ is a gross population for an orbital $\text{μ}$ in the basis set employed defined according to $$q_{\text{μ}}=P_{\text{μ}\text{μ}}+\sum _{\begin{array}{c}\nu \ne \text{μ}\end{array}}{P}_{\text{μ}\nu }{S}_{\text{μ}\nu }$$ where $P_{\text{μ}\text{ν}}$ and $S_{\text{μ}\text{ν}}$ are the elements of density matrix and overlap matrix, respectively (see overlap integral). In the Hückel molecular orbital theory (where $S_{\text{μ}\nu }={\delta }_{\text{μ}\nu }$), $q_{\text{μ}}=n_{\text{μ}}{P}_{\text{μ}\text{μ}}$, where $n_{\text{μ}}$ is the number of electrons in the $\text{MO}\text{μ}$.