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