where { y i ≠ j } is the set of all natural variables of Φ except y j that are held as constants. This yields expressions for various thermodynamic parameters in terms of the derivatives of the potentials with respect to their natural variables. These equations are known as equations of state since they specify parameters of the thermodynamic state. If we restrict ourselves to the potentials U (Internal energy), F (Helmholtz energy), H (Enthalpy) and G (Gibbs energy), then we have the following equations of state (subscripts showing natural variables that are held as constants):