@property<EOL><INDENT>def U(self):<DEDENT>
return property_molar_to_mass(self.Um, self.MW) if (self.Um is not None) else None<EOL>
r'''Internal energy of the chemical at its current temperature and
        pressure, in units of [J/kg].

        This property requires that :obj:`thermo.chemical.set_thermo` ran
        successfully to be accurate.
        It also depends on the molar volume of the chemical at its current
        conditions.