Tables for
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.3, pp. 121-122

Section Processed intensities, their positions and processing information

B. H. Tobya*

aNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA
Correspondence e-mail: Processed intensities, their positions and processing information

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The data items in these categories are as follows:

(a) Part of PD_DATA [Scheme scheme13]

(b) PD_PROC_INFO [Scheme scheme14]

(c) PD_PROC_LS [Scheme scheme15]

The pdCIF dictionary distinguishes between values that are measured directly and values that are derived from these observations. For example, in a constant-wavelength instrument, diffraction intensities are recorded as a function of [2\theta]. One may derive d-space values from the [2\theta] values using the value of the wavelength and corrections for the [2\theta] zero-point error and the sample displacement. One may also derive a new set of data points from the observations, for example by summing adjacent data points when the increment between the data points is much smaller than is warranted by the peak widths. For peak searching and other non-quantitative purposes, the diffraction intensities may be smoothed or otherwise modified. Note that the unprocessed measurement values are retained using the data items _pd_meas_*. Since the original measurements are still available, modifications like these do not result in the loss of the original data. In fact, by placing processed values in multiple blocks, a single CIF may contain measurements that have been processed in more than one way.

It is good practice to use the _pd_proc_info_author_* and _pd_proc_info_datetime data items. It is also a good idea to describe how the measurements were processed using _pd_proc_info_data_reduction.

The _pd_proc_* data items in this list may be used to calibrate the [2\theta] or energy values of the data. These are defined in the items _pd_proc_2theta_corrected, _pd_proc_2theta_range_*, _pd_proc_d_spacing, _pd_proc_energy_*, _pd_proc_recip_len_Q and _pd_proc_wavelength.

When corrections, scaling or other processing, such as averaging or smoothing, are applied to the intensities, the results are stored using the _pd_proc_intensity_* data items. Note that if the number of data points does not change, it might be most convenient to include the processed intensities in the same loop as the observed values. This is not always possible, so these items can be placed in a separate loop if there is no longer a one-to-one correspondence between the [2\theta] or energy positions for the _pd_proc_intensity_* values and the _pd_meas_counts_* or _pd_meas_intensity_* values.

For energy-dispersive measurements, the incident spectrum must be determined for normalization. This can be recorded using _pd_proc_intensity_incident. For other types of normalization, _pd_proc_intensity_norm should be used.

For full-pattern fitting, there is a series of _pd_proc_ls_* data items for recording settings and results. For example, agreement factors can be recorded using the _pd_proc_ls_prof_*_factor data items. Some data items may be included in the loop(s) containing the measured or the processed data: _pd_proc_ls_weight specifies the weight assigned to each point and _pd_proc_intensity_bkg_calc specifies the fitted background. Note that background values are usually generated by extrapolation from fixed values set during the refinement or are determined from a function that is fitted to the observations, and occasionally both are used together. The function that has been fitted can be described using _pd_proc_ls_background_function, while fixed points are listed using _pd_proc_intensity_bkg_fix. If sections of the pattern are not fitted, this is indicated using _pd_proc_info_excluded_regions.

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