Regards, I'm developing a dynamic model of an evaporator, but I don't understand  how to get the derivative of density of liquid respect to the pressure. I understand that this expression is from an EoS, but what I don't understand is how to get it. I'm using MATLAB for this modeling, and also REFPROP to get the state of the fluid (R134a), also I know that REFPROP have this parameter  but I want to understand how to get this parameter.