Determining the density for binary mixtures is much more difficult than for pure gases, since the composition is added to the dependencies of the system variables. Therefore, I decided to implement a separate property for binary gas mixtures. The return values of this function are, on the one hand, the partial densities of the two components and the density of the gas phase, and on the other hand, the first derivatives of these three quantities according to gas pressure, capillary pressure and temperature. Therefore, the eigen-matrix <3,1> for value and the eigen-matrix <3,3> for dValue are suitable as data types for the return values. Since I always need all nine derivatives, I decided to calculate them all with a single call.
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