  Jammed disordered packings of non-spherical particles show significant
variation in the packing density as a function of particle shape for a given
packing protocol. Rotationally symmetric elongated shapes such as ellipsoids,
spherocylinders, and dimers, e.g., pack significantly denser than spheres over
a narrow range of aspect ratios, exhibiting a characteristic peak at aspect
ratios of $\alpha_{\rm max}\approx 1.4-1.5$. However, the structural features
that underlie this non-monotonic behaviour in the packing density are unknown.
Here, we study disordered packings of frictionless dimers in three dimensions
generated by a gravitational pouring protocol in LAMMPS. Focusing on the
characteristics of contacts as well as orientational and translational order
metrics, we identify a number of structural features that accompany the
formation of maximally dense packings as the dimer aspect ratio $\alpha$ is
varied from the spherical limit. Our results highlight that dimer packings
undergo significant structural changes as $\alpha$ increases up to $\alpha_{\rm
max}$ manifest in the reorganisation of the contact configurations between
neighbouring dimers, increasing nematic order, and decreasing local
translational order. Remarkably, for $\alpha>\alpha_{\rm max}$ our metrics
remain largely unchanged, indicating that the peak in the packing density is
related to the interplay of structural rearrangements for $\alpha<\alpha_{\rm
max}$ and subsequent excluded volume effects with unchanged structure for
$\alpha>\alpha_{\rm max}$.
