8 "Liquid" model for thermal motion

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Here "laue" uses a different model, called a liquid model, to represent the thermal motion. Atoms are put down NOT near lattice sites, but at distances from their neighbors equal to the nominal lattice constant plus or minus a random amount of rms amplitude sigma liquid. For the preset display the sigma liquid is 0.4, giving an electron density plot indistinguishable by eyeball from that of the preceding crystal model, preset 7. (Click calculate a number of times for one preset, and then again for the other, to make a fair comparison.) Now reduce the sigma to 0.04 and look carefully at the diffraction pattern. The diffuse scattering looks similar to the crystal case.

But look carefully at the Bragg peaks. Now they are no longer cleanly defined, but are broadened by the disorder. The two models are distinctly different though the electron densities seemed identical. The issue is one of long range order which is preserved by the crystal model with atoms always near well defined lattice sites, but which is destroyed in the liquid model.