TIL that, when following the Slater(-Condon) rules, when calculating the energy's of the wavefunction of an atom/molecule, in using full CI expansion, the energy in the coverage between the single excited Slater Determinant and a higher excited Slater determinant (given as <phi(a,b;r,s) | H | phi(a;r)>) doesn't equal 0, because of some serious shenanigans. See below. To understand this, it has to be noted that normally orbitals are orthonormal to each other, making normally the coverage between two spinorbitals is given by epsilon * KroneckerDelta(a,r). (KroneckerDelta means: if a=r, KD