 Two-electron Coulomb integral

The electrostatic repulsion integral $J_{pq}=\iint |{\varphi }_p(1){|}^2|{\varphi }_q(2){|}^2{r}_{12}^{-1}d1d2$ in atomic units. For a normalized hydrogenic 1s orbital of exponent $\zeta$, the same-orbital value is $5\zeta /8$. It is a two-electron integral, unlike a diagonal Hückel one-electron parameter.