Agreement factors

The effects of the 7/2 correction on the final refinements of these routine data sets was negligible. There was no significant change in the final agreement factors and... 

Refinements of the use of the experimental data obtained using these reagents, e.g. determination of the agreement factor R in pseudo-contact models and methods of calculating experimental errors, have also been reported. 

The Hellmann-Feynman constraint has been applied successfully to the exocyclic fluorine, carbon, and nitrogen atoms in tetrafluoroterephthalonitrile (1,4-dicyano-2,3,5,6-tetrafluorobenzene). Charge balance is achieved without deterioration of the least-squares agreement factors, though the resulting changes in the density maps are very small (Hirshfeld 1984) (see chapter 5). 

Mallinson et al. (1988) have performed an analysis of a set of static theoretical structure factors based on a wave function of the octahedral, high-spin hexa-aquairon(II) ion by Newton and coworkers (Jafri et al. 1980, Logan et al. 1984). To simulate the crystal field, the occupancy of the orbitals was modified to represent a low-spin complex with preferential occupancy of the t2g orbitals, rather than the more even distribution found in the high-spin complex. The complex ion (Fig. 10.14) was centered at the corners of a cubic unit cell with a = 10.000 A and space group Pm3. Refinement of the 1375 static structure factors (sin 8/X < 1.2 A 1) gave an agreement factor of R = 4.35% for the spherical-atom model with variable positional parameters (Table 10.12). Addition of three anharmonic thermal... 

For both of these renins multiple copies of the molecules have been independently defined in the x-ray analysis and shown to have very similar structures. These x-ray structures were refined to final agreement factors and correlation coefficients of 0.19 and 0.91 for human renin at 2.8 A resolution and... 

Maps from the simulated dynamic data and from the static simulated data are hardly distinguishable from one another or from the theoretical density mapped directly from the extended basis wave function (see Figure 3). Furthermore, the statistical agreements factors are excellent [/ (F)] = 3.1, 3.0, 3.2, and 3.1%o, respectively, for the static and 75 K, 150 K, and room temperature simulated data. This shows that the pseudoatom model effectively recovers the theoretical electron density that comes from the dynamic structure factors. A full report, including a refinement against real room temperature structure factors of H3P04, is given in references 19 and 20. 

There are a number of factors that can affect the final value of the agreement factor R, such as the amount of thermal motion. Some other factors are the quality and size of the crystal used for data collection. Too small a specimen will produce weak reflections that are difficult to measure accurately. A soft crystal is readily susceptible to physical distortions that will result in attenuations in the magnitudes of the reflections. Some molecules, usually large ones, have some small variation in their conformation, such as rotations about single bonds, from cell to cell in the crystal that produce a positional disorder and have the effect of increasing the R factor. 

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