Defined by: \[U^{\unicode[Times]{x3C3} } = U - U^{\unicode[Times]{x3B1} }- U^{\unicode[Times]{x3B2} }= U - V^{\unicode[Times]{x3B1} }\ \frac{U_{\text{m}}^{\unicode[Times]{x3B1} }}{V_{\text{m}}^{\unicode[Times]{x3B1} }} - V^{\unicode[Times]{x3B2} }\ \frac{U_{\text{m}}^{\unicode[Times]{x3B2} }}{V_{\text{m}}^{\unicode[Times]{x3B2} }}\] where V α and V β satisfy the condition V α + V β = V, the total volume of the system. (U m α V m α) and (U m β V m β) are the energy densities in the two bulk phases where U m α and U m β are the mean molar energies and V m α and V m β are the mean molar volumes of the two phases.