This work establishes the utility of the exchange charge density \(q_\mathit x(\mathbf{r})\) , evaluated within the local density approximation (LDA), for exploring many-electron multinuclear systems at the subatomic scale and for characterizing noncovalent Lewis acid–base interactions in crystals. The LDA affords this quantity in a form readily computable from the electron density \(\rho(\mathbf{r})\) , its gradient \(\nabla \rho(\mathbf{r})\) , and its Laplacian \(\nabla^2 \rho(\mathbf{r})\) . As model systems, we consider the crystalline forms of Appel’s salt and 1-(5-bromopyridin-2-yl)ethan-1-one, which display a rich array of Lewis-type interatomic interactions, notably halogen, chalcogen, and tetrel bonding. For these compounds, LDA-derived \(q_\mathit x(\mathbf{r})\) resolves internal atomic shells and distinguishes between regions of condensed and decondensed electron-pair density within valence shells—regions that include covalent bonds, lone pairs, and electron-deficient sites—thereby revealing the nucleophilic and electrophilic loci of Lewis bases and acids, respectively. Overall, \(q_\mathit x(\mathbf{r})\) emerges as a practical and chemically meaningful descriptor for quantum crystallography, supramolecular chemistry, and crystal engineering. In addition, a distinction is drawn between the local (de)concentration of electronic charge within the electron cloud, as expressed by \(\nabla^2 \rho(\mathbf{r})\) , and the (de)condensation of electron-pair density within the many-electron cloud, as described by \(q_\mathit x(\mathbf{r})\) . Importantly, an expression for the local exchange charge density \(q_\mathit x(\mathbf{r})\) in terms of the bilocal Fermi hole density \(h_{\mathit x}(\mathbf{r}, \mathbf{r}')\) is presented. This expression can be decomposed into three terms, each written in integral-kernel form as an operator acting on \(h_{\mathit x}(\mathbf{r}, \mathbf{r}')\) : a distributional operator involving \(\delta(\mathbf{r}-\mathbf{r}')\) ; a dipolar operator in which \(\nabla_{\mathbf{r}}\) acts on \(h_{\mathit x}(\mathbf{r}, \mathbf{r}')\) and is contracted, via a dot product, with \((\mathbf{r}-\mathbf{r}')/ |\mathbf{r}-\mathbf{r}'|^{3}\) ; and an operator in which \(\nabla^{2}_{\mathbf{r}}\) acts on \(h_{\mathit x}(\mathbf{r}, \mathbf{r}')\) under the Coulomb kernel \({1}/{|\mathbf{r}-\mathbf{r}'|}\) . Together, these terms encode how local, directional, and curvature features of the Fermi hole shape the (de)condensation of electron-pair density within the many-electron cloud.
