For a free electron gas with Fermi wavevector kF, the energy U of volume O is5 h2kF

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This contribution to the energy of the condensed phase generates no interatomic force since U is independent of the atomic positions. However, its contribution is significant: metallic cohesive energy and bulk moduli are correct to within an order of magnitude. Consider­ation of this term gives some justification for ignoring the cohesive energy and bulk modulus in fitting a potential, and fitting shear moduli, vacancy, or surface energies instead. The discrepancy is absorbed by a putative free electron contribution which does not contribute to the interatomic atomic forces in a con­stant volume ensemble calculation.