  This work proposes a classification algorithm based on the radical Voronoi
tessellation to define the Widom delta, supercritical gas-liquid coexistence
region, of polyatomic molecules. In specific, we use a weighted mean-field
classification method to classify a molecule into either gas-like or
liquid-like. Classical percolation theory methods are adopted to understand the
generality of the structural transition and to locate the Widom delta. A
structural analysis on various supercritical fluids shows that the proposed
method detects the influence of the attractive interaction on the structural
transition of supercritical fluids. Moreover, we demonstrate that the
supercritical gas-liquid coexistence region of water overlap with the ridges of
the response function maxima. From the pressure-temperature relation, a
three-parameter corresponding state theorem is derived, which states that the
fraction of gas-like molecules of a substance is equal to that of another if
their reduced pressure, reduced temperature and the critical compressibility
factor are the same.
