Two-electron exchange integral
= Two-electron exchange integral
{title2=$K_{pq}$}
For real <spatial orbitals>, $K_{pq}=\iint\phi_p(1)\phi_q(1)r_{12}^{-1}\phi_q(2)\phi_p(2)\,d1\,d2$ in <atomic units>. It enters the energy of a <Slater determinant> through antisymmetry and becomes small for well-separated localized orbitals.