  As in the available literature there are still misconceptions about powder
diffraction phenomena observed for small nanocrystals ($D<10$ nm), we propose
here a systematic and concise review of the involved issues that can be
approached by atomistic simulations. Most of phenomenological tools of powder
diffraction can be now verified constructing realistic atomistic models,
following their thermodynamics and impact on the diffraction pattern. The
models concern small cuts of the perfect lattice as well as relaxed
nanocrystals also with typical strain and faults, proven by experiments to
approximate the real nanocrystals. The discussed examples concern metal
nanocrystals. We describe the origin of peak shifts that for cuts of the
perfect lattice are mostly due to multiplying of broad profiles by steep slope
factors -- atomic scattering and Lorentz. The Lorentz factor embedded in the
Debye summation is discussed in more detail. For models relaxed with realistic
force fields and for the real nanocrystals the peak shift additionally includes
contribution from surface relaxation which enables development of an
experimental {\it{in situ }} method sensitive to the state of the surface. Such
results are briefly reviewed. For small fcc nanocrystals we discuss the
importance and effect of multiple (111) cross twinning on the peak shift and
height. The strain and size effects for the perfect multitwinned clusters --
decahedra and icosahedra, are explained and visualised. The manuscript proposes
new methods to interpret powder diffraction patterns of real, defected
(twinned) fcc nanoparticles and points to unsuitability of the Rietveld method
in this case.
