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Below $0.02~\mathrm{a.u.}$, the lack of experimental points precludes a direct comparison, but we find a large deviation from linear behavior.   One possible explanation is that bound effects break down the linear response (Linhard) approximation that was useful to interpret the different regimes (and crossover) for velocities above $0.02~\mathrm{a.u.}$.   Probing this regime experimentally would be rather  difficult, especially to disentangle it from nuclear stopping effects, but if this is confirmed it would be an unexpected new regime possibly related to bound effects. In any case, the combination of experimental and theoretical results show that the limit $v\to 0$ is intricate for metals as it is for insulators \cite{Artacho_2007, Lim_2015} where analogous band and gap threshold effects have been found.