NONUNIFORM REFERENCE MODEL FOR MAXIMUM-ENTROPY DENSITY RECONSTRUCTIONS FROM DIFFRACTION DATA

Diffraction experiments provide information on the Fourier components of microscopic density distributions in crystals. To obtain the spatia l densities themselves, an inverse Fourier problem has to be solved. T he procedure is complicated by the presence of noise and incompletenes s of the data. The application of the maximum-entropy (MaxEnt) princip le was a breakthrough in density reconstruction, allowing high-quality density maps to be obtained without involving any a priori informatio n concerning what the reconstructed density should look like. In this work, a procedure is proposed that incorporates a priori (e.g. theoret ical) information into MaxEnt reconstructions of spin density distribu tions. It allows, on the one hand, the evaluation of the existing dens ity models and, on the other, the precise investigation of what new in formation the experiment brings. Unlike traditional parameter-refineme nt techniques, the new method does not impose any strict constraints o n the density to be reconstructed and is thus much more flexible. At t he same time, it suppresses artifacts and yields high-quality density maps. The advantages of the new methods are illustrated by an example of spin density reconstruction based on real polarized neutron diffrac tion data.