  One of the most remarkable results to emerge from heavy-ion collisions over
the past two decades is the striking regularity shown by particle yields at all
energies. This has led to several very successful proposals describing particle
yields over a very wide range of beam energies, reaching from 1 A GeV up to 200
A GeV, using only one or two parameters. A systematic comparison of these
proposals is presented here. The conditions of fixed energy per particle,
baryon+anti-baryon density, normalized entropy density as well as percolation
model are investigated. The results are compared with the most recent chemical
freeze-out parameters obtained in the thermal-statistical analysis of particle
yields. The sensitivity and dependence of the results on parameters is analyzed
and discussed. It is shown that in the energy range above the top AGS energy,
within present accuracies, all chemical freeze-out criteria give a fairly good
description of the particle yields. However, the low energy heavy-ion data
favor the constant energy per particle as a unified condition of chemical
particle freeze-out. This condition also shows the weakest sensitivity on model
assumptions and parameters.
