Single scattering approximations (the kinematic and the weak phase object approx- imation), because of their simplicity, are perhaps the most attractive formulations for structure analysis by electron diffraction. In these approximations, the diffracted wave function is linearly related to the object potential. The validities of these approximations are, however, limited to very thin crystals at low resolution. In materials science the failure of the single scattering approximations and the impor- tance of the dynamical scattering effect have been well accepted. In biological science, the large unit cell size and the low atomic number (e.g. protein crystals) have lead some to believe that the dynamical scattering effect is insignificant for crystallographic analysis. Contrary to this belief, the number of dynamically interacting beams increases with the crystal unit cell size. It is important to note here that the dynamical scattering effect depends on the values of the excitation errors and on the magnitudes of the Fourier coefficients of the crystal potential.
Electron Diffraction of Frozen, Hydrated Protein Crystals
