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> Something for you to chew on: how is it that the electrons of the protein, > which are presumably not in phase with each other nor in exactly the same > place in their orbitals from unit cell to unit cell (maybe they are?) when > they scatter the photons, they result in interference? What are the chances > that the scattering electrons are exactly in the same place as the > electrons in another unit cell, or of the same phase? And would they not > need to be in the same place to sub-angstrom precision to scatter > coherently? They do not need to be in the same place to sub-angstrom precision. The situation is easier to understand when you think about a crystal composed of small molecules or even better - atoms (e.g. a Silicon crystal). If all electrons (pointlike particles) were in the same place, atomic scattering factor would be a flat function against the scattering angle (the atomic scattering factor would be a constant function (putting aside polarization considerations)) and we would not observe a falloff of intensities for large scattering angles. By the way, thinking in terms of orbitals can be a bit misleading, because orbitals are like components of a vector: in different coordinate systems one gets different sets of components. What has a physical meaning is a wave function. The wave function can be decomposed into orbitals (localized, delocalized etc.) in many different ways. Andrzej ______________________ Andrzej Olczak Institute of General and Ecological Chemistry Technical University of Lodz Zeromskiego 116 90-924 Lodz Poland ______________________