1. In DFT, why do most texts ignore/don't mention the attractive exchange "force" between two antiparallel spins electrons?

    The repulsive "exchange" force of two same spins electrons are always mentioned to correct the Coulombic repulsion, which is logical. But books and youtube lectures often ignore the fact that there is an exchange "force" between two antiparallel spins that are attractive. And given that electrons are fermions, with antisymmetric spin wavefunction and symmetric space wavefunction, shouldn't we be emphasizing more on this force?

  2. Is the correlation energy the Coulombic interaction that was ignored when we were considering the indistinguishability of the two electrons? If so, it should be present in both same spin electrons and different spin electrons, right? In the context of DFT, it seems that we always consider the correlation repulsion of different spin electrons.
  • $\begingroup$ To support your thoughts I want to point to the correlation between the electrons spin and the electrons magnetic dipole moment. In my feeling the interaction between these tiny magnets is more important for the structure of the electron shells when the electric charge. $\endgroup$ – HolgerFiedler Jan 23 '18 at 21:09
  • $\begingroup$ @HolgerFiedler The tiny magnets you mentioned here are referring to the spin of the electrons, right? If so, then it should be already incorporated into the problem when we consider the antisymmetric properties of the wavefunction, isn't it? (Where to obey Pauli Exclusion Principle, the space wavefunction and spin wavefunction both took part such that we get the total wavefunction to be antisymmetrical) Or are you referring to the electromagnetic interaction? In that case, I'm not really sure how would the problem change. Still hoping someone one to clear me up on this question. Hahaha $\endgroup$ – KYS Jan 24 '18 at 12:04

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