  There is strong evidence of the failure of hydrodynamic simulations of the
quark-gluon plasma (QGP) to reproduce data on the elliptic flow of particles in
relativistic collisions of $^{238}$U nuclei at the BNL Relativistic Heavy Ion
Collider (RHIC). We demonstrate that this failure is caused by an inappropriate
implementation of well-deformed ions, such as $^{238}$U, in the hydrodynamic
framework. Past studies have identified the deformation of the nuclear surface
with that of the nuclear volume, though these are different concepts. In
particular, a volume quadrupole moment can be generated by both a surface
hexadecapole and a surface quadrupole moment. This feature was so far neglected
in the modeling of heavy-ion collisions, and is particularly relevant for
nuclei like $^{238}$U, which is both quadrupole- and hexadecapole-deformed.
With rigorous input from Skyrme density functional calculations, we show that
correcting for such effects in the implementation of nuclear deformations in
hydrodynamic simulations restores agreement with BNL RHIC data. This brings
consistency to the results of nuclear experiments across energy scales, and
demonstrates the impact of the hexadecapole deformation of $^{238}$U on
high-energy collisions.
