Editing Free electron model (section)

=== Magnetic response ===
According to the [[Bohr–Van Leeuwen theorem]], a classical system at thermodynamic equilibrium cannot have a magnetic response. The magnetic properties of matter in terms of a microscopic theory are purely quantum mechanical. For an electron gas, the total magnetic response is [[paramagnetism|paramagnetic]] and its [[magnetic susceptibility]] given by{{Cn|date=April 2024}} 
:<math>\chi=\frac{2}{3}\mu_0\mu_\mathrm{B}^2g(E_\mathrm{F}),</math>
where <math display="inline">\mu_0</math> is the [[vacuum permittivity]] and the <math display="inline">\mu_{\rm B}</math> is the [[Bohr magneton]]. This value results from the competition of two contributions: a [[Diamagnetism|diamagnetic]] contribution (known as [[Diamagnetism#Theory|Landau's diamagnetism]]) coming from the orbital motion of the electrons in the presence of a magnetic field, and a paramagnetic contribution (Pauli's paramagnetism). The latter contribution is three times larger in absolute value than the diamagnetic contribution and comes from the electron [[Spin (physics)|spin]], an intrinsic quantum degree of freedom that can take two discrete values and it is associated to the [[electron magnetic moment]].