  We study the low-temperature behavior of a simple cluster-crystal forming
system through simulation. The phase behavior is found to be hybrid between the
Gaussian core and penetrable sphere models. The system additionally exhibits a
series of reentrant crystallization and melting loops as well as critical
isostructural transitions between crystals of different occupancy. Due to the
creation and annihilation of lattice sites, the system further shows an unusual
and intriguing softening upon compression.
