Another method to determine the coexistence points is based on the Helmholtz potential minimum principle, which states that in a system in diathermal contact with a heat reservoir T = T R , D F = 0 and D 2 F > 0 , namely at equilibrium the Helmholtz potential is a minimum. Since, like g ( p , T ) , the molar Helmholtz function f ( v , T ) is also a potential function whose differential is, d f = ∂ ∂ v f | T d v + ∂ ∂ T f | v d T = − − p d v − − s d T , this minimum principle leads to the stability condition ∂ ∂ 2 f / ∂ ∂ v 2 | T = − − ∂ ∂ p / ∂ ∂ v | T > 0 . This condition requires that at any stable state of the system the function f is strictly convex, namely that in its vicinity the curve lies on or above its tangent. Moreover, for those states the previous stability condition for the pressure is necessarily satisfied as well.