  We investigate the impact of the addition of nanoparticles (NP) on the
fragility of a model glass-forming polymer melt by molecular dynamics
simulations. We find significant changes in fragility for nanoparticle volume
fractions $\phi$ exceeding $\approx$ 5%, where fragility changes correlate with
the inverse variance of the magnitude of the Debye-Waller factor $<u^2>$, a
measure of local "stiffness" fluctuations. We also confirm the validity of the
Buchenau relationship between $<u^2>$ and the structural relaxation time $\tau$
for all $\phi$ and polymer-NP interaction types.
