  We apply the concept of dynamic propensity to a simple kinetically
constrained model of glass formers, the two-vacancy assisted triangular lattice
gas, or (2)-TLG. We find that the propensity field, defined in our case as the
local root-mean square displacement averaged over the ensemble of trajectories
with identical initial configurations, is a good measure of dynamical
heterogeneity. This suggests a configurational origin for spatial fluctuations
of the dynamics, but just as in the case of atomistic systems, we find that
propensity is not correlated to any simple structural property. We show instead
that certain extended clusters of particles connected to vacancies correlate
well with propensity, indicating that these are the fundamental excitations of
the (2)-TLG. We also discuss time-correlations and the correlation between
configurations within the propensity ensemble.
