While calculating the binding energies in metallic systems, we take into account the exchange and correlation effects. Basic understanding of these effects is necessary for exact calculations.
Dear Tahir - both effects are very common and observable (or important for) e.g. X-ray photoelectron spectroscopy (XPS).
Generally speaking, the exchange interaction (or the exchange energy) is a quantum mechanical effect on identical particles, it is importnat e.g. for the formation of chemical bonds: Simply speaking, the exchange of an electron from one atom to another may lead to the formation of an attractive force, i.e. a bonding between the atoms. In terms of quantum mechanics, it alters the expectation value of the distance when the wave functions (or orbitals) of two indistinguishable particles overlap.
The correlation energy is also of quantum mechanical origin: You may not treat an electron as a single particle when it is present in a metal, where it interacts with many many other electrons in the band structure. So if you manipulate one electron, you will affect all the others. A typical example is maybe photoemission: If you create a core hole, all the remaining electrons will bind more closely to the core, because the attractive forces will increase, leading to an increase of the related binding energies.
Check the following linksfor an overview and more details:
http://en.wikipedia.org/wiki/Exchange_interaction
http://en.wikipedia.org/wiki/Electronic_correlation
and/or textbooks on quantum mechanics for even more details.
Hope this helps, Dirk
Dear Tahir - both effects are very common and observable (or important for) e.g. X-ray photoelectron spectroscopy (XPS).
Generally speaking, the exchange interaction (or the exchange energy) is a quantum mechanical effect on identical particles, it is importnat e.g. for the formation of chemical bonds: Simply speaking, the exchange of an electron from one atom to another may lead to the formation of an attractive force, i.e. a bonding between the atoms. In terms of quantum mechanics, it alters the expectation value of the distance when the wave functions (or orbitals) of two indistinguishable particles overlap.
The correlation energy is also of quantum mechanical origin: You may not treat an electron as a single particle when it is present in a metal, where it interacts with many many other electrons in the band structure. So if you manipulate one electron, you will affect all the others. A typical example is maybe photoemission: If you create a core hole, all the remaining electrons will bind more closely to the core, because the attractive forces will increase, leading to an increase of the related binding energies.
Check the following linksfor an overview and more details:
http://en.wikipedia.org/wiki/Exchange_interaction
http://en.wikipedia.org/wiki/Electronic_correlation
and/or textbooks on quantum mechanics for even more details.
Hope this helps, Dirk
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