MEED (maximum‐entropy electron density) is a program package to calculate the electron‐density distribution from a set of structure‐factor data by the maximum‐entropy method. MEED is an upgraded version of the original maximum‐entropy program, MEMTARO, which was used in the first study to use the maximum‐entropy method (MEM) on silicon [Sakata & Sato (1990). Acta Cryst. A46, 263–270]. MEED is applicable to any space group and can cope with both single‐crystal and powder X‐ray diffraction data, whereas MEMTARO can only after modification. Another upgraded feature is the speed of calculation. By employing a new algorithm, MEED is much faster than MEMTARO for the same calculation. Computing time depends on various factors, such as the number of reflection data, accuracy of data and the number of symmetry operations. It is estimated that MEED is typically 100 times faster than MEMTARO. In an extreme case like the beryllium powder‐data case, MEED is 600 times faster than MEMTARO. MEED is coded in Fortran77 for both a scaler computer, FACOM M780, and a vector computer, FACOM VP2600, which are mainframe computers at the Computation Center of Nagoya University. MEED enables the electron‐density distribution to be calculated for any crystalline material, with a fine pixel size, e.g. with 128 × 128 × 128 pixels to a unit cell, provided that accurate diffraction data are available. MEED can overcome, to some extent, one of the biggest drawbacks of MEM analysis, the vast computing time required.
Single crystal neutron diffraction analysis of photo-exposed (3-cyanopropyl-d2(alpha,alpha))-[(R)-1-phenylethylamine-d11]bis(dimethylglyoximato-d14)cobalt(III) was carried out in order to clarify the mechanism of the crystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms bonded to the C1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as --CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimensions of the crystal were gradually changed. After 7 d exposure the change became insignificantly small. The structure was analyzed by neutron diffraction. The 3-cyanopropyl group was transformed to the 1-cyanopropyl group such as --CD(CN)C(H1/2,D1/2)2CH3 with retention of the single-crystal form. This indicates that one of the D atoms bonded to C1 migrates to either position bonded to C2. The other atoms of the complex remained unchanged. These results indicate that photoisomerization proceeded in two steps: the 3-cyanopropyl group was isomerized to the 2-cyanopropyl group in the first place and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl group. Moreover, it was made clear that the second-step isomerization was irreversible, since one of the D atoms was retained. The disordered structure at C2 is estimated to be caused by the interconversion between the 1-cyanopropyl group produced and its dehydrogenated olefin after the photoisomerization.
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