I am just starting to learn Quantum Chemistry for my research. I am currently reading Modern Quantum Chemistry by Szabo and I am a little confused by a concept. The two-electron ground state energy is: $$E_0 = <\psi|H|\psi> = \sum_a[a|h|a]+\frac{1}{2}\sum_a\sum_b{[aa|bb]-[ab|ba]}$$ where the fir...