Superancillary equations have been developed for the recommended (by NIST) multiparameter equations of state (EOS) for all 147 pure fluids in NIST REFPROP 10.0. These superancillary equations represent the orthobaric densities and saturation pressure of the EOS as a function of temperature by Chebyshev expansions to an accuracy better than the iterative calculations in REFPROP and are hundreds to thousands of times faster to evaluate than a full iterative solution of Maxwell’s criteria. The C++ code required to develop and test the superancillary equations is provided as open-source material. The methodology is straightforwardly extensible to new multiparameter EOS, establishing a new paradigm for the evaluation of vapor–liquid equilibria for pure fluids.

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