For the ratio dJmeas/Jmeas we insert the planimetering values. The characteristic temperature (O = 543 °K) of chromium at 310°K was obtained from the work of Wilson et al. (1966).The following results were obtained from the calculations: This result is about 2.8 times larger than the one calculated from the discontinuity of the elastic constants (Wilson et al., 1966 A two-dimensional polynomial method is described which allows rapid evaluation of bond-scattering factors in a structure factor least-squares program. Correlation effects between electron population parameters are analyzed. It is concluded that simultaneous refinement of 2s2 and three 2p2 one-center populations is not possible. Similarly, for the y axis being along a bond AB, the distributions px(A)p~ (B) and p~(A)px(B) are identical and only the sum of the corresponding population parameters can be refined. A variety of models is discussed, in which the number of parameters is reduced by selective elimination. In the two-center model all terms but those describing 'long' bonds are included. In the symmetry-restricted two-center model all bonds are required to have mm symmetry, while in the onecenter model all two-center terms are neglected. Finally, a full table of symmetry restrictions on population parameters and a brief discussion of thermal motion treatment are given.

No abstract

A least-squares refinement of x-ray diffraction data has been developed in which the parameters are the populations of atomic orbital products describing the molecular electron density distribution. The procedure is applied to alpha-oxalic acid dihydrate and cyanuric acid. Complementary structural information obtained by neutron diffraction has been used. In the absence of complementary information, the method allows simultaneous determination of structural and charge-density parameters. There is an indication of a migration of charge from the ppi to the psigma orbitals in both molecules.

Three‐dimensional models of complex molecules, electron density contours, and text may be generated graphically and in stereo on a color television monitor by a 4096 word mini‐computer and prototype display system assembled from commercially available components. Interactive features include translation and rotation of the asymmetric unit for packing studies, rotation of the stereo image, and molecular geometry calculations.

Trial models of the rigid molecule, spirodienone, were oriented and positioned in the crystallographic cell by means of visual packing analysis. Because there are two molecules in the asymmetric unit, 12 positional parameters were adjusted independently to obtain visually 'reasonable' packing arrangements. One such arrangement refined both by varying the 12 parameters with respect to repulsion energies as well as by full-matrix group refinement with respect to observed structure factors, which are reported elsewhere. The R values of the partially refined models were 42 % and 35 % respectively, at which point individual atom least-squares refinement was begun yielding a final R value of 4.8 %.