10.3: Correlation - conditional probability
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Hartree-Fock theory does not properly describe correlation. In the Copenhagen Interpretation, the squared modulus of the wavefunction gives the probability of finding a particle in a given place. The many-body wave function gives the N-particle distribution function, i.e. |Φ(r1,...,rN)|2 is the probability density that particle 1 is at r1,..., and particle N is at rN.
But when trying to work out the interaction between electrons, what we want to know is the probability of finding an electron at r, given the positions of all the other electrons {ri}. This implies that the electron behaves quantum mechanically when we evaluate its wavefunction, but as a classical point particle when it contributes to the potential seen by the other electrons.