def _derivDimensional(self, rho, T):
        """Calcule the dimensional form or Helmholtz free energy derivatives

        Parameters
        ----------
        rho : float
            Density, [kg/m³]
        T : float
            Temperature, [K]

        Returns
        -------
        prop : dict
            Dictionary with residual helmholtz energy and derivatives:

                * fir, [kJ/kg]
                * firt: ∂fir/∂T|ρ, [kJ/kgK]
                * fird: ∂fir/∂ρ|T, [kJ/m³kg²]
                * firtt: ∂²fir/∂T²|ρ, [kJ/kgK²]
                * firdt: ∂²fir/∂T∂ρ, [kJ/m³kg²K]
                * firdd: ∂²fir/∂ρ²|T, [kJ/m⁶kg]

        References
        ----------
        IAPWS, Guideline on an Equation of State for Humid Air in Contact with
        Seawater and Ice, Consistent with the IAPWS Formulation 2008 for the
        Thermodynamic Properties of Seawater, Table 7,
        http://www.iapws.org/relguide/SeaAir.html
        """
        if not rho:
            prop = {}
            prop["fir"] = 0
            prop["firt"] = 0
            prop["fird"] = 0
            prop["firtt"] = 0
            prop["firdt"] = 0
            prop["firdd"] = 0
            return prop

        R = self._constants.get("R")/self._constants.get("M", self.M)
        rhoc = self._constants.get("rhoref", self.rhoc)
        Tc = self._constants.get("Tref", self.Tc)
        delta = rho/rhoc
        tau = Tc/T

        ideal = self._phi0(tau, delta)
        fio = ideal["fio"]
        fiot = ideal["fiot"]
        fiott = ideal["fiott"]
        fiod = ideal["fiod"]
        fiodd = ideal["fiodd"]

        res = self._phir(tau, delta)
        fir = res["fir"]
        firt = res["firt"]
        firtt = res["firtt"]
        fird = res["fird"]
        firdd = res["firdd"]
        firdt = res["firdt"]

        prop = {}
        prop["fir"] = R*T*(fio+fir)
        prop["firt"] = R*(fio+fir-(fiot+firt)*tau)
        prop["fird"] = R*T/rhoc*(fiod+fird)
        prop["firtt"] = R*tau**2/T*(fiott+firtt)
        prop["firdt"] = R/rhoc*(fiod+fird-firdt*tau)
        prop["firdd"] = R*T/rhoc**2*(fiodd+firdd)
        return prop
Calcule the dimensional form or Helmholtz free energy derivatives

        Parameters
        ----------
        rho : float
            Density, [kg/m³]
        T : float
            Temperature, [K]

        Returns
        -------
        prop : dict
            Dictionary with residual helmholtz energy and derivatives:

                * fir, [kJ/kg]
                * firt: ∂fir/∂T|ρ, [kJ/kgK]
                * fird: ∂fir/∂ρ|T, [kJ/m³kg²]
                * firtt: ∂²fir/∂T²|ρ, [kJ/kgK²]
                * firdt: ∂²fir/∂T∂ρ, [kJ/m³kg²K]
                * firdd: ∂²fir/∂ρ²|T, [kJ/m⁶kg]

        References
        ----------
        IAPWS, Guideline on an Equation of State for Humid Air in Contact with
        Seawater and Ice, Consistent with the IAPWS Formulation 2008 for the
        Thermodynamic Properties of Seawater, Table 7,
        http://www.iapws.org/relguide/SeaAir.html