Neither CAPE-OPEN nor RefProp supports total overall properties derivatives, which may be needed for dynamic simulation, especially for state variable transformation. It was therefore decided that these types of calculations should be implemented on the Modelica side and not in the C-interface. For a pure fluid, the calculations are straight-forward and there are publications available (Thorade et al, 2013). For mixtures they are more complicated (Li, 1955). The difficult part is when several phases coexist. In that case they are currently calculated numerically.

For an example of how derivativs are calculated see MultiPhaseMixture.Templates.ExternalTwoPhaseMixture.Auxilary.Wrapper.density_derh_p_X


[1] James C. M. Li, Clapeyron Equation for MultiComponent Systems, The Journal of Chemical Physics volume 25. number 3 september. 1956.

[2] Mathis Thorade and Ali Saadat, Partial derivatives of thermodynamic state properties for dynamic simulation, Environ Earth Sci 70:3497–3503. 2013

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