An account of some computing experiences

Abstract

Experience is summarized in the use of fractional shifts of scale, temperature and distance parameters, values of r = 2:w(IFo]2--JFcl2)2/2:wl•or 4, the behavior of temperature factors and a threedimensional Patterson superposition program in the determination of a number of structures.

The crystal and molecular structures of strontium tartrate trihydrate and calcium tartrate tetrahydrate

“…The atomic scattering factors for O, C and Ca 2+ were taken from International Tables for X-ray Crystallography (1962). found in other tartrates (Beevers & Hughes, 1941;Stern & Beevers, 1950;Sadanaga, 1950;Parry, 1951;Sprenkels, 1956;van Bommel & Bijvoet, 1958;Ambady & Kartha, 1968). The two halves of the molecule, each consisting of a carboxyl group, a tetrahedral carbon and a hydroxyl oxygen, are individually planar.…”

“…The strategy finally adopted was to sandwich least-squares refinements between successive electron density Fourier syntheses. The thermal parameter of an incorrectly placed atom invariably increased with successive least-squares cycles (Rossmann, Jacobson, Hirshfeld & Lipscomb, 1959). Such atoms are thereafter omitted from succeeding electron density Fourier syntheses.…”

Crystal structure of the dimer of rhodium chloride 1,5-cyclooctadiene

“…There is no structural explanation for this. Of course, the high thermal parameter might indicate that atom 4' is misplaced slightly from minimum position (Rossmann et al, 1959). Because of this, and because the data of Table 1 lead to rings with obvious distortions, further refinements were attempted in which particular carbon atoms, such as 4', or particular groups of carbon atoms, were deliberately displaced from the positions of Table 1 to 'more reasonable' positions.…”

Topochemistry. XI. The crystal structures of methyl-m- andp-bromocinnamates

“…A trial model was established from the two Patterson projections P(xz) and P(yz) and refined by a combination of (Qo-Qe) syntheses and of least-squares computations. The latter were based on the diagonal-approximation method (Rossmann, Jacobson, Hirshfeld & Lipscomb, 1959), which has been described, together with the weighting technique employed here, in part V of this series (Rabinovich & Schmidt, 1964). In the later least-squares cycles all the atoms except the methyl carbon were assigned anisotropic temperature factors; in addition, all but the methyl-group hydrogens were inserted on the basis of C-H bond lengths of 1.08 A and normal bond angles, and kept fixed during the refinement.…”