# A question about definition of Fermi energy

Wikipedia states the definition of Fermi energy as for "a system of non-interacting fermions". If we have to assume free electrons in a solid behave this way before we are able to calculate Fermi energy, how can Pauli exclusion be justified (because electrons are non-interacting)? Can Fermi energy be similarly defined for electrons confined to a single atom?

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Nuclear physicists have no trouble defining a Fermi energy for interacting nucleons, it's just more complicated than the non-interacting case. See the first chapter of Walecka's book among others. –  dmckee Mar 20 '12 at 0:55
I assume you mean "because electrons are interacting" in your parenthetical. This is justified by Landau Fermi-liquid theory, and it applies to crystal quasi-particles. There is a similar qualitative idea in atoms, but it isn't used much (as far as I know). –  Ron Maimon Mar 20 '12 at 4:00
The definition "a system of non-interaction fermions", i.e. a Fermi gas exists in a metallic solid after the Pauli principle has been taken into account. So roughly: 'free' electrons in a metal + Pauli = Fermi gas. For this Fermi gas you can define a Fermi energy. –  Alexander Mar 20 '12 at 8:39
I am seeking for an answer to the less emphasized question too : Can Fermi energy be defined for electrons confined to a single atom? –  Hiran Mar 20 '12 at 19:02