Like Wilson (1964), Renon & Prausnitz (1968) began with local composition theory, but instead of using the Flory–Huggins volumetric expression as Wilson did, they assumed local compositions followed
with a new "non-randomness" parameter α. The excess Gibbs free energy was then determined to be
Unlike Wilsons equation, this can predict partially miscible mixtures. However, the cross term, like Wohls expansion, is more suitable for  than , and experimental data is not always sufficiently plentiful to yield three meaningful values, so later attempts to extend Wilsons equation to partial miscibility (or to extend Guggenheims quasichemical theory for nonrandom mixtures to Wilson's different-sized molecules) eventually yielded variants like UNIQUAC.