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== Correlation ==
There are several forms of correlation:
There are several parameterizations of the electron gas correlation:<ref name=pw92>{{cite journal | title = Accurate and simple analytic representation of the electron-gas correlation energy | author = John P. Perdew and Yue Wang | journal = Phys. Rev. B | volume = 45 | pages = 13244 - 13249 | year = 1992 | url = http://link.aps.org/abstract/PRB/v45/p13244 | doi = 10.1103/PhysRevB.45.13244 }}</ref>▼
* Wigner <ref name=wigner>{cite journal | title = On the Interaction of Electrons in Metals | author = E. Wigner | journal = Phys. Rev. | volume = 46 | pages = 1002 - 1011 | year = 1934 | url = http://link.aps.org/abstract/PR/v46/p1002 | doi = 10.1103/PhysRev.46.1002 }}</ref><ref>{{cite journal | title = Theory of Metal Surfaces: Charge Density and Surface Energy | author = N. D. Lang and W. Kohn | journal = Phys. Rev. B | volume = 1 | pages = 4555 - 4568 | year = 1970 | url = http://link.aps.org/abstract/PRB/v1/p4555 | doi = 10.1103/PhysRevB.1.4555 }}</ref>
* Vosko-Wilk-Nusair (VWN) <ref>{{cite journal | title = Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis | author = S. H. Vosko, L. Wilk and M. Nusair | journal = Can. J. Phys. | volume = 58 | pages = 1200 | year = 1980 }}</ref>
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* Lee-Yang-Parr (LYP) <ref name=lyp>{{cite journal | title = Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density | author = Chengteh Lee, Weitao Yang, and Robert G. Parr | journal = Phys. Rev. B | volume = 37 | pages = 785 - 789 | year = 1988 | url = http://link.aps.org/abstract/PRB/v37/p785 | doi = 10.1103/PhysRevB.37.785 }} </ref>
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Wigner correlation is gotten by using equally spaced electrons and applying perturbation theory.<ref name=wigner />
VWN, PZ and PW92 are fit to a [[Quantum Monte Carlo|quantum Monte Carlo]] calculation<ref>{{cite journal | title = Ground State of the Electron Gas by a Stochastic Method | author = D. M. Ceperley and B. J. Alder | journal = Phys. Rev. Lett. | volume = 45 | pages = 566 - 569 | year = 1980 | url = http://link.aps.org/abstract/PRL/v45/p566 | doi = 10.1103/PhysRevLett.45.566}}</ref> of the electron gas at varying densities.<ref name=pw92 />
LYP is based on data fit to the helium atom.<ref name=lyp />
== References ==
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