Abstract
For non-centrosymmetric crystals containing both positive and negative scatterers of neutrons, the symbolic addition procedure has been tested as a means of obtaining phases of neutron reflexions. Test calculations were done for two structures, α-resorcinol and methyl GAG. 2HCl. H2O, which have already been studied by neutron diffraction using conventional methods. It is found that this procedure yields the phase angles with reasonable accuracy. The average error in phases for α-resorcinol was 14° and the Fo Fourier synthesis with these phases revealed the position of all the positive scatterers and three out of six hydrogen atoms. The average error in methyl GAG. 2HCl. H2O was 40° and only 10 positively scattering atoms out of 16 could be clearly seen in the Fourier map. For both the structures an Fo map was found to be superior to an E map. When the contribution of the negative scatterers to the total neutron scattering is less than 25%, it has been shown that the symbolic addition method will apply for crystals containing up to 100 atoms per unit cell.
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