Consider a system in isothermal (T =const) isochoric (the volume V =const) condition. For these conditions, the Helmholtz free energy ⁠ F ( T , V , N ) ⁠ measures the “useful” work obtainable from a system. It is a functions of the temperature T, the volume V and the amounts of chemical components N j (usually measured in moles), N is the vector with components N j. For the ideal systems, F = R T ∑ ∑ i N i ( ln ⁡ ⁡ ( N i V ) − − 1 + μ μ i ⊖ ⊖ ( T ) R T ) .