  Simulations of electron and positron channelling in a crystalline undulator
with a small amplitude and a short period (A Kostyuk, Phys. Rev. Lett. 110
(2013) 115503) were repeated by V G Bezchastnov, A V Korol and A V Solov'yov
(J. Phys. B: At. Mol. Opt. Phys. 47 (2014) 195401)for the same parameter set
but using another computer code, MBN Explorer, which implements a different
model of projectile scattering by crystal atoms. The authors of the latter
paper claim that their approach, in contrast to the one of the former paper,
allows them to observe short-period undulator oscillations in plots of
simulated trajectories. In fact, the undulator oscillations become visible on
trajectory segments which have a small amplitude of channelling oscillations.
This is equally true for both approaches. The claim of Bezchastnov et al. that
their model is "more accurate" is unfounded. Moreover, there are indications of
severe mistakes in their calculations: the phases of undulator oscillations in
the trajectories obtained with MBN Explorer are inconsistent with each other
and with well established properties of forced oscillations.
