## Wikipedia - Forza di Coulomb (it) Coulomb repulsion

https://doi.org/10.1351/goldbook.CT07013
The potential energy component corresponding to the electrostatic interaction between each pair of charged particles: $V=\frac{1}{4\: \pi\: \varepsilon_{0}}\ \sum _{i}\sum _{j \lt i}e_{i}\ e_{j}\ \Delta r_{ij}$ where $$\varepsilon _{0}$$ is the @[email protected] of a vacuum, $$\Delta r_{ij}$$ is the distance between the two particles, and $$e_{i}$$ and $$e_{j}$$ are the charges on particles $$i$$ and $$j$$. In @[email protected] theory, the electrostatic repulsion between the two electrons occupying the orbitals $$\Psi _{i}$$ and $$\Psi _{j}$$. In the @[email protected]–Fock method, the mean @[email protected] repulsion is determined by the value of the @[email protected] $J_{ij}=\int \int \Psi _{i\text{*}}\left(\mathbf{r}_{1}\right)\ \Psi _{i}\left(\mathbf{r}_{1}\right)\ \left(\frac{e^{2}}{r_{12}}\right)\ \Psi _{j\text{*}}\left(\mathbf{r}_{2}\right)\ \Psi _{j}\left(\mathbf{r}_{2}\right) \ \mathrm{d}\mathbf{r}_{1} \ \mathrm{d}\mathbf{r}_{2} = \lt ij|ij \gt$