General Information

Miscellaneous
- Title: Enter a descriptive alphanumeric title.
- Space Group: First, click on the selector button. Then choose the appropriate crystal system, next the lattice type, and finally select the desired space group itself. Note that the text box cannot be edited directly. All selections of space group are through the selector button.
- Data Type:
  
  Basic indicates that a complete structure (not a substructure) will be phased. A single set of data should be supplied, and any Bijvoet pairs present in the data will be averaged. If Fa values have been derived from MAD data, this option can also be used to locate the sites for an anomalously scattering substructure. If Fa values are being used, DO NOT apply the Bayesian correction to the weak intensities, but DO increase the numbers of reflections and invariants to use by 5-6 times the default values for Basic applications.
  
  SIR indicates that a substructure will be phased using isomorphous difference data. Data files will be needed for the native and derivative structures. Alternatively, this option can be used to base the phasing procedure on the dispersive differences between two wavelengths of MAD data but, in practice, this approach has had a low success rate.
  
  SAS indicates that a substructure will be phased using anomalous difference data. A single data set containing pairs of Bijvoet differences will be required. Data for each of the wavelengths from a MAD experiment can be examined individually using this option. In most cases, SAS applications to the wavelength with the peak anomalous difference provide a successful way to locate the positions of the anomalous scatterers.
- Radiation: Choose the type of radiation used (CU, MO, or SYNCHROTRON).
Asymmetric Unit (ASU)
- Contents: Enter the empirical formula corresponding to the asymmetric unit. Denote the elements as they appear in the Periodic Table (i.e. enter Se and not SE for selenium). Exact atom counts are not required for the lighter elements, and C, N, and O can be combined if desired. It is not necessary to include solvent, but a reasonable approximation for the solvent content certainly would not hurt. The following approximation is suggested for a peptide with R residues: C 5R, N 1.2R, O1.5R, and H 8R.
  
  For SAS or SIR data, make sure that you enter only the substructure formula and not the formula for the entire structure.
  
  The contents of this field are used to determine Nu, the number of unique non-H atoms in the asymmetric unit. The default values of several critical parameters (numbers of reflections, invariants, peaks, and cycles) are based on the value of Nu.
Cell Constants and Cell Errors
- Errors are not mandatory; they are simply included when a SHELXL "ins" file is written.
Anomalous Dispersion Correction

The wavelength of the diffraction experiment, the identify of the anomalously scattering element, and the values of f' and f'' MUST be supplied for the native data set if the SAS data type is selected. In SAS cases, the same element should be included above in the asymmetric unit contents and ALSO in the native ASU contents supplied on the Create Es screen. This information MAY be included for Basic and SIR data. This information is used during the normalization process and, although experience has shown that exact values of f' and f'' are not critical, it is desirable to be as accurate as possible. It is best to determine these values experimentally at the time of the diffraction experiment. Theoretical values can be found at Ethan Merritt's anomalous scattering web site, but it should be remembered that theory does not give accurate values near absorption edges. The following values are a good guess for the Se K-edge anomalous peak: f' = -7.4 and f'' = 5.9.