# Le Chatelier's principle

Le Chatelier's principle describes the stability of a system in thermodynamic equilibrium[1][2]:

In response to small deviations away from equilibrium, the system will change in a manner that restores equilibrium.

This translates to conditions on the second derivatives of thermodynamic potentials such as entropy, $S(U,\ldots)$. For instance, the entropy is a concave function of its arguments such as internal energy. Thus, one has

$\frac{\partial^2 S}{\partial U^2} \geq0\ .$

Similarly, specific heats can be shown to be positive definite.