To derive the MP E0(2), we set c(0) = abij in (16.12); the sum over excited states

Question:

To derive the MP E0(2), we set ψc(0) = Фabij in (16.12); the sum over excited states s 0 in (16.12) is replaced by a quadruple sum over i, j, a, and b that produces all possible determinants that contain two excited spin-orbitals and that represent different states.
(a) For this quadruple sum, explain why we want the limits for i and j to be as in (16.13); explain why Фabij = 0 if a = b, and explain why we want the limits for a and b to be as in (16.13).
(b) Use the Condon-Slater rules to evaluate (Фabij | H'|Ф0) and show that (16.13) follows