  The pressure-temperature phase diagram of a one-component system, with
particles interacting through a spherically symmetric pair potential is
studied. It is shown that if the pair potential allows for a discontinuous
reduction of the volume of the system when pressure is increased (at zero
temperature), then the phase diagram obtained has many ``water like''
characteristics. Among these characteristics the negative thermal expansion and
expansion upon freezing in some range of pressure, and the existence of many
different crystaline structures are clearly observable in numerical
simulations. This implies that complex phase diagrams are not necessarily
originated in complex microscopic interactions.
