  Particles traveling in aligned crystals at small angles w.r.t.
crystallographic axes or planes are principally steered by the continuous
Lindhard potential. This interaction conserves the energy E, the longitudinal
momentum p_parallel, the transverse energy of the particle E_perp and is
elastic concerning the crystal quantum state. At high enough energy the
particle motion is quasi-classical. The time-dependent fluctuations of the
positions of the atoms or of the electrons of the crystal create a residual
potential, on which the particle can scatter. This interaction does not
conserve the previous quantities and is inelastic for the crystal. We compare
its treatments with the classical binary collision model and with a
phenomenological quantum model. The classical dechanneling rate is estimated to
be several ten per cent larger than the quantum one. The influence of
correlated vibrations of neighboring atoms is discussed.
