  In the framework of topological constraint theory, network glasses are
classified as flexible, stressed--rigid, or isostatic if the number of atomic
constraints is smaller, larger, or equal to the number of atomic degrees of
freedom. Here, based on molecular dynamics simulations, we show that sodium
aluminosilicate glasses exhibit a flexible-to-stressed--rigid transition driven
by their composition. This transition manifests itself by a loss of atomic
mobility and an onset of internal atomic stress. Importantly, we find that the
flexible-to-rigid (i.e., loss of internal flexibility) and
unstressed-to-stressed transitions (i.e., onset of internal stress) do not
occur at the same composition. This suggests that the isostatic state (i.e.,
rigid but unstressed) is achieved within a window rather than at a threshold
composition.
