Electron correlation vs electron exchange

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Discussion Overview

The discussion centers on the distinction between electron correlation and electron exchange, exploring their definitions, implications in quantum mechanics, and their mathematical representations. The scope includes theoretical concepts and mathematical reasoning related to many-electron systems.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant asks about the differences between electron correlation and electron exchange, specifically regarding their relation to electron spin and charge.
  • Another participant explains that in a classical context, two neutral particles are uncorrelated, but when they become indistinguishable in a quantum context, electron exchange becomes relevant due to the exclusion principle.
  • The same participant notes that when particles are charged, their evolution becomes correlated, indicating a more complex interaction.
  • A later reply introduces the concept of the Hartree product wavefunction and explains that achieving exchange symmetry requires considering all permutations, leading to a Slater matrix as a representation of non-correlated electrons.
  • This participant also mentions that a fully correlated wavefunction cannot be represented as a single determinant but as an infinite sum, referred to as the full configuration interaction (CI) limit.

Areas of Agreement / Disagreement

Participants present different aspects of electron correlation and exchange, but there is no consensus on a singular definition or understanding of the concepts, indicating that multiple competing views remain.

Contextual Notes

The discussion involves complex quantum mechanical principles and mathematical representations that may depend on specific definitions and assumptions about electron interactions.

Rzbs
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TL;DR
What is difference between electron correlation and electron exchange?
What is the difference between electron correlation and electron exchange?
Which of them is due to the spin of electrons and which is due to charge of electrons?
 
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All of this can be easily understood mathematically using the so-called correlation functions, but I'll try to explain with words.

First, consider two neutral (non interacting) classical particles. In this case the two particles basically ignore each others so the evolution in time of each particle is totally independent (uncorrelated) of the other.

Now let's add a little bit of quantum mechanics: suppose the two particles are now indistinguishable. This is a purely quantum mechanical phenomenon and it changes the system. The two particles are still non-interacting (so their time evolution is still independent from each other) but now you get all the quantum mechanical effects that follow from indistinguishability (ex. exclusion principle). This is roughly the electron-exchange.

Finally suppose the particles are charged. Now the evolution of one particle is dependent of the other in a pretty complicated way and we say the particles are correlated.

Schematically you can think it this way:
classical view + exchange + correlation = real qm description.

In some cases the exchange term and the correlation terms are unified into the so-called exchange-correlation term.
 
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Let me add this:
The most classical approximation to a many electron problem is via a Hartree product wavefunction. To get exchange symmetry, you have to consider all permutations and end up with a Slater matrix. This is the most general wavefunction describing somehow non-correlated electrons (each electron sees only the average field of the other electrons). The fully correlated wavefunction cannot be expressed as a single determinant but only as a (in principle infinite) sum of these. This is called the full CI (configuration interaction) limit.
 
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