  Emerging coherent X-ray scattering patterns of single-particles have shown
dominant morphological signatures in agreement with predictions of the
scattering model used for conventional protein crystallography. The key
question is if and to what extent these scattering patterns contain volumetric
information, and what model can retrieve it. This contribution is Part 2 out of
two reports, in which we seek to clarify the assumptions of some different
regimes and models of X-ray scattering and their implications for single
particle imaging. In Part 1, basic concepts and existing scattering models
along with their implications for nanocrystals, and also the misconception of
using Diffraction Theory for volumetric scattering were addressed. Here in Part
2, specific challenges ahead of single particle imaging are addressed.
Limitations of the conventional scattering model in the test case of a sphere,
schemes for improving this common scattering model, ambiguities in meaning and
properties of "density map" and non-classical effects, the crucial role of
electromagnetic boundary conditions, the uniqueness of X-ray scattering inverse
problems, and additional vulnerabilities of phase retrieval and its relation
with "resolution" are discussed. We raise concerns about the unverified use of
the common scattering model of protein crystallography for arbitrary objects,
and also leaving fundamental questions such as meaning/uniqueness/properties of
the sought 3D profiles to phase retrieval algorithms.
