When a metallic system is heated from absolute zero, not every electron gains an energy k B T as equipartition dictates. Only those electrons in atomic orbitals within an energy range of 3 2 k B T of the Fermi level are thermally excited. Electrons, in contrast to a classical gas, can only move into free states in their energetic neighbourhood. The one-electron energy levels are specified by the wave vector k through the relation ϵ ϵ ( k ) = ℏ ℏ 2 k 2 / 2 m with m the electron mass. This relation separates the occupied energy states from the unoccupied ones and corresponds to the spherical surface in k -space. As T → → 0 the ground state distribution becomes: