  The relative roles of multiple electron scattering and in-molecule free-space
propagation in transmission electron microscopy of small molecules are
discussed. It is argued that while multiple scattering tends to have only a
moderate effect in this case, the in-molecule Fresnel diffraction is likely to
be significant due to the shallow depth of focus under the relevant
experimental conditions. As a consequence, diffraction tomography based on the
first Born or first Rytov approximation represents a more suitable method for
the reconstruction of three-dimensional distribution of the electrostatic
potential in this context, compared to conventional computed tomography which
is intrinsically based on the projection approximation. A simplified method for
diffraction tomography is proposed and tested on numerically simulated
examples.
