Neglecting the spin–orbit interaction, the wavefunction for the two electrons can be written as Ψ Ψ = Ψ Ψ 0 Ψ Ψ s , where we have split the wavefunction into spatial and spin parts. As mentioned above, Ψ Ψ needs to be antisymmetric, and so the antisymmetry can arise either from the spin part or the spatial part. There are 4 possible spin states for this system: