  We suggest a simple model to describe polyamorphism in single-component
fluids using a maximum-valence approach. The model contains three types of
interactions: i) atoms attract each other by van der Waals forces that generate
a liquid-gas transition at low pressures, ii) atoms may form covalent bonds
that induce association, and iii) bonded atoms attract or repel each other
stronger than non-bonded atoms, thus generating liquid-liquid separation. As an
example, we qualitatively compare this model with the behavior of liquid sulfur
and show that condition (iii) generates a liquid-liquid phase transition in
addition to the liquid-gas phase transition.
