The differential Fourier method and the method of steepest descents have been adapted for iterative refinement of centrosymmetric crystal structures using the high speed automatic calculator built by J. Lyons and Company Limited. The methods have been applied to the refinement of the crystal structure of nicotinamide, which has 9 crystallographically independent atoms and 939 observed hid reflexions. The differential refinement for a structure of this complexity takes 35 rain.per iteration and the steepest-descents refinement 40 rain., including the recording of the individual calculated structure-factor values on magnetic tape.
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