= Electron correlation
= Correlated electrons
{synonym}
<Electron correlation> concerns the dependence between electronic degrees of freedom that cannot be captured by treating each <electron> as moving independently. In a quantum-chemical use of the term, correlation usually means effects beyond the best single-<Slater determinant> <Hartree-Fock method>; that method already includes exchange associated with fermionic antisymmetry.
The distinction matters when comparing approximations. <Dynamic electron correlation> describes additional instantaneous avoidance even when one electronic configuration dominates. <Static electron correlation> becomes important when several configurations have comparable significance, as during some bond-dissociation processes. A correlation <energy> is therefore defined relative to a stated reference calculation, and the physical need for multiple configurations is different from merely allowing a single orbital to change shape.
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