  Coherent diffraction imaging (CDI) of single molecules at atomic resolution
is a major goal for the x-ray free electron lasers (XFELs). However, during an
imaging pulse, the fast laser-induced ionization may strongly affect the
recorded diffraction pattern of the irradiated sample. The radiation tolerance
of the imaged molecule should then be investigated 'a priori' with a dedicated
simulation tool. The continuum approach is a powerful tool for modeling the
evolution of irradiated large systems consisting of more than a few hundred
thousand atoms. However, this method follows the evolution of average
single-particle densities, and the experimentally recorded intensities reflect
the spatial two-particle correlations. The information on these correlations is
then inherently not accessible within the continuum approach. In this paper we
analyze this limitation of continuum models and discuss the applicability of
continuum models for imaging studies. We propose a formula to calculate
scattered intensities (including both elastic and inelastic scattering) from
the estimates obtained with a single-particle continuum model. We derive this
formula for systems under conditions typical for CDI studies with XFELs.
