🚀 go-pugleaf

Thank you for the explanation.  I think I'm beginning to get a glimmer.
Let me try out my own rewording for size:

1)  The exchange energy is not simply another name for the Pauli exclusion
    principle.  Rather, it is correction to the classical calculation for
    the electrostatic energy, and arises from the antisymmetry
    properties of the wave function for an ensemble of electrons.
    Coulomb's formula applied to an antisymmetric distribution yields
    (classical electrostatic energy) + (exchange energy) + lower order terms.
    (Would it be correct to say it is due to the need to treat an
    ensemble of electrons with Fermi-Dirac statistics rather than Boltzman
    statistics?)

2)  The exchange energy affects the way electrons fill up energy levels in a
    solid, again modifying classical expectations.  For iron, it turns out
    the minimum energy configuration is with the electon "gas" having aligned
    spins, and the bound electrons having spins aligned too, but aligned
    opposite to the free electrons.

3)  There is no simple qualitative explanation of why iron should have this
    sort of minimum energy state, while copper, for instance, doesn't.