Abstract
WHEN we attempt to infer the positions of the atoms in a crystalline structure from measurements of X-ray diffraction, all methods reduce in principle to a matter of 'trial and error'. The investigator tentatively places the atoms in certain positions, using judgment and past experience to set up an arrangement which does not conflict with interatomic distances and probable groupings. He then calculates how such an arrangement would diffract X-rays, and makes a comparison with what is observed. If there is no correspondence, he must try again. By eliminating possibilities and by adjustments of any structure which shows hopeful signs of checking with observations, he finally arrives (if successful) at an arrangement which he can regard as established. All this involves an immense amount of calculation. He may, for example, make measurements of a thousand diffracted beams. For each model, he must calculate the intensities of diffraction and compare it with what is observed. When the structure is complex, containing many atoms, there are so many possible permutations and combinations of the atomic positions that considerable courage and perseverance are necessary. A structure of a new type, such as a complex silicate or an organic molecule such as a sugar, has often meant one or two years of hard work and the accumulation of drawers full of calculations. The labour is well repaid, because so often a new structure casts quite a new light on an important chemical problem. Any method, however, of reducing the labours of calculation, and so saving the time of the expert, is of value.
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References
Nature, 152, 411 (1943).
Nature, 153, 81 (1944).
Nature, 143, 678 (April 22, 1939); 149, 470 (April 25, 1942).
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BRAGG, L. Lightning Calculations With Light*. Nature 154, 69–72 (1944). https://doi.org/10.1038/154069a0
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DOI: https://doi.org/10.1038/154069a0
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