Section 3.2.2.3. Experimental measurements on the crystal

The categories describing experimental conditions are as follows:

The data items in these categories are as follows:

(a) EXPTL

(b) EXPTL_CRYSTAL

(c) EXPTL_CRYSTAL_FACE

The bullet () indicates a category key. Where multiple items within a category are marked by a bullet, they must be taken together to form a compound key.

The EXPTL category is rather broadly named, but in practice is used to record details about any absorption correction applied and, using _exptl_special_details, any other details of the experimental work prior to intensity measurement not specifically described by other data items (e.g. _exptl_crystal_preparation).

The data items in the EXPTL_CRYSTAL category are designed to record details of experimental measurements on the crystal or crystals used. Since it is usually the case that just one crystal is used throughout the experiment, the category is presented as if it comprises non-looped data names. However, details of a number of crystals may be looped together, in which case _exptl_crystal_id is used to identify the different crystals and acts as the category key.

When different crystals are used to collect diffraction intensities, it is likely that the intensities collected from each crystal would need to be scaled by different factors, as recorded by the DIFFRN_SCALE_GROUP category and the _diffrn_refln_scale_group_code used for each individual reflection. In these circumstances, it would be good practice to use matching values of _diffrn_refln_scale_group_code and _exptl_crystal_id, although this is not mandatory.

Note that the F(000) value, which is often calculated as the integer number of electrons in the crystal unit cell, may contain dispersion contributions and is more properly calculated aswhere and are, respectively, the real and imaginary parts of the scattering factors at and the sum is taken over each atom in the unit cell.

The crystal colour may be given as free text using the data item _exptl_crystal_colour. Alternatively, the standardized names developed by the International Centre for Diffraction Data to classify specimen colours may be constructed from the items _exptl_crystal_colour_lustre, *_modifier and *_primary, each of which has a restricted set of specific values.

The EXPTL_CRYSTAL_FACE category records details of the crystal faces. The faces are defined by Miller indices and their perpendicular distances from the centre of rotation of the crystal may be recorded in millimetres. Absolute orientations with respect to the goniometer angle settings may also be recorded. The category is currently constructed in a way that cannot distinguish between multiple crystals.