## atomic charge

https://doi.org/10.1351/goldbook.AT06994
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 @[email protected] of $$A$$ and $$q_{\text{A}}$$ is the @[email protected] assigned to $$A$$. The method of calculation of $$q_{\text{A}}$$ depends on the choice of the scheme of partitioning @[email protected] In the framework of the Mulliken population analysis $$q_{\text{A}}$$ is associated with the so-called gross atomic population: $$q_{\text{A}}=\sum q_{\unicode[Times]{x3BC}}$$, where $$q_{\unicode[Times]{x3BC}}$$ is a gross population for an orbital $$\unicode[Times]{x3BC}$$ in the @[email protected] 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 @[email protected]). In the Hückel @[email protected] 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 $$\text{MO}\,\unicode[Times]{x3BC}$$.
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1924 [Terms] [Paper]