  Within the shoving model of the glass transition, the relaxation time and the
viscosity are related to the local cage rigidity. This approach can be extended
down to the atomic-level in terms of the interatomic interaction, or potential
of mean-force. We applied this approach to both real metallic glass-formers and
model Lennard-Jones glasses. The main outcome of this analysis is that in
metallic glasses the thermal expansion contribution is mostly independent of
composition and is uncorrelated with the interatomic repulsion: as a
consequence, the fragility increases upon increasing the interatomic repulsion
steepness. In the Lennard-Jones glasses, the scenario is opposite: thermal
expansion and interatomic repulsion contributions are strongly correlated, and
the fragility decreases upon increasing the repulsion steepness. This framework
allows one to tell apart systems where "soft atoms make strong glasses" from
those where, instead, "soft atoms make fragile glasses". Hence, it opens up the
way for the rational, atomistic tuning of the fragility and viscosity of widely
different glass-forming materials all the way from strong to fragile.
