Those variables are not independent. The allowed states are defined by an equation of state relating those three variables: F ( T , P , ν ν ) = 0. The function F depends on the material under consideration. The specific internal energy stored internally in the sample, per unit of mass, will then be another function U ( T , P , ν ν ) of these state variables, that is also specific of the material. The total internal energy in the sample then will be M U ( T , P , ν ν ) .