  We solve a long-standing experimental discrepancy of NH$_3$BH$_3$, which---as
a molecule---has a threefold rotational axis, but in its crystallized form at
room temperature shows a fourfold symmetry about the same axis, creating a
geometric incompatibility. To explain this peculiar experimental result, we
study the dynamics of this system with ab initio Car-Parrinello molecular
dynamics and nudged-elastic-band simulations. We find that rotations, rather
than spatial static disorder, at angular velocities of 2 rev/ps---a time scale
too small to be resolved by standard experimental techniques---are responsible
for the fourfold symmetry.
