  A characteristic property of many soft matter systems is an ultrasoft
effective interaction between their structural units. This softness often leads
to complex behavior. In particular, ultrasoft systems under pressure
demonstrate polymorphism of complex crystal and quasicrystal structures.
Therefore, it is of interest to investigate how different can be the structure
of the fluid state in such systems at different pressures. Here we address this
issue for the model liquid composed of particles interacting through the
harmonic-repulsive pair potential. This system can form different crystal
structures as the liquid is cooled. We find that, at certain pressures, the
liquid exhibits unusual properties, such as the negative thermal expansion
coefficient. Besides, the volume and the potential energy of the system can
increase during crystallization. At certain pressures, the system demonstrates
high stability against crystallization and it is hardly possible to crystallize
it on the timescales of the simulations. To address the liquid's structure at
high pressures, we consider the scaled pair distribution function (PDF) and the
bond-orientational order (BOO) parameters. The marked change happening with the
PDF, as pressure increases, is the splitting of the first peak which is caused
by the appearance of non-negligible interaction with the second neighbors and
the following rearrangement of the structure. Our findings suggest that
non-trivial effects, usually explained by different interactions at different
spatial scales, can be observed also in one-component systems with simple
one-length-scale ultrasoft repulsive interactions.
