  We perform molecular dynamics simulations to study lithium dynamics in a
model of LiPO$_3$ glass at temperatures below the glass transition. A
straightforward analysis of the ionic trajectories shows that lithium diffusion
results from jumps between sites that are basically unmodified on the time
scale of the lithium ionic relaxation. This allows us a detailed identification
and characterization of the sites. The results indicate that the number of
lithium sites is only slightly bigger than the number of lithium ions so that
the fraction of vacant sites is very limited at every instant. Mapping the
ionic trajectories onto series of jumps between the sites provides direct
access to lithium jump dynamics. For each site, we determine the mean residence
time $\tau_s$ and the probability $p_s^b$ that a jump from this site to another
site is followed by a direct backjump. While a broad distribution $G(\lg
\tau_s)$ shows that different sites feature diverse lithium dynamics, high
values of $p_s^b$ give direct evidence for back-and-forth jumps. We further
study how the local glass structure and the local energy landscape affect
lithium jump dynamics. We observe substantial effects due to the energy
landscape, which are difficult to capture within single-particle approaches.
