The Helmholtz energy-volume-temperature surface (AVT), Legendre-transformed from the previous UVS model. An isothermal tangent line touches liquid (red cross) and vapor (green cross) states that can coexist in contact because they both satisfy the criteria for mutual phase equilibrium. The red (unstable) region has a curvature in the volume direction opposite to that of the blue and yellow regions, thus making the yellow-red boundary show points of inflection

Helmholtz energy is plotted positively downward in this drawing, which makes the slope of the tangent line positive:

Iowa State University