11''

A = e^{2} \int (1 / r_{12}) ψ_{a} (1) ψ_{b}^{⋆} (1) ψ_{a}^{⋆} (2) ψ_{b} (2) d V_{1} d V_{2}

${A = e^2 \int (1/r_{12}) \psi_a(1) \psi_b^\star (1) \psi_a^\star (2) \psi_b(2) dV_1 dV_2}$