This parameter is always real according to the second law of thermodynamics. In fact the second law of thermodynamics can be expressed by several postulates. The most elementary of them in mathematical terms is the statement of convexity of the fundamental equation of state, i.e. the hessian matrix of the specific energy expressed as function of specific volume and specific entropy: ( e v v e v s e v s e s s ) is defined positive. This statement corresponds to the two conditions: { e v v > 0 e v v e s s − − e v s 2 > 0