  A structural order parameter for disordered configurations is defined, based,
not on local topologies, but on the degree of local restraint imposed on each
atom. This restraint parameter provides a clear distinction between a strong
liquid (SiO2) and a fragile liquid (a binary Lennard-Jones mixture) without
reference to dynamics. Where the fragile liquid exhibits an abrupt transition
from unrestrained to restrain below the melting point, molten silica is highly
constrained even at equilibrium. We determine the temperature dependence of the
average number of particles restrained per pinned particle and consider the
feature of the potential energy landscape responsible for determining the
fragility.
