  Effective pair interactions with a soft-repulsive component are a well-known
feature of polymer solutions and colloidal suspensions, but they also provide a
key to interpret the high-pressure behaviour of simple elements. We have
computed the zero-temperature phase diagram of four different model potentials
with various degrees of core softness. Among the reviewed crystal structures,
there are also a number of non-Bravais lattices, chosen among those observed in
real systems. Some of these crystals are indeed found to be stable for the
selected potentials. We recognize an apparently universal trend for unbounded
potentials, going from high- to low-coordinated crystal phases and back upon
increasing the pressure. Conversely, a bounded repulsion may lead to
intermittent appearance of compact structures with compression and no eventual
settling down in a specific phase. In both cases, the fluid phase repeatedly
reenters at intermediate pressures, as suggested by a cell-theory treatment of
the solids. These findings are of relevance for soft matter in general, but
they also offer fresh insight into the mechanisms subtended to solid
polymorphism in elemental substances.
