This is an EOS of the “generalized cubics” category. In comparison to simpler EOS, it has 2 additional parameters for the density dependence of the attraction term, and 2 parameters for its temperature dependence. For the latter, a correlation with the acentric factor is available.
The equation gives good vapour pressure curves and density data for many compounds. Note: If there are 3 real roots for the volume, the smallest one may be in a physically forbidden range.
adjustable parameters: 6
 P. H. Salim and M. A. Trebble, Fluid Phase Equilib. 65 (1991) 59–71.
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