checkCIF procedure

PROC-NAME: CRYSS02 Type_3

PURPOSE: To check that the values of _exptl_crystal_size_* are not larger than expected.

PROCEDURE:
(1) IF _exptl_crystal_size_min > 0.6 && _diffrn_radiation_type != neutron
issue ALERT B
   "Alert B The value of _exptl_crystal_size_min is > 0.6"
(2) IF _exptl_crystal_size_mid > 0.8 && _diffrn_radiation_type != neutron
issue ALERT B
   "Alert B The value of _exptl_crystal_size_mid is > 0.8"
(3) IF _exptl_crystal_size_max > 1.0 && _diffrn_radiation_type != neutron
issue ALERT B
   "Alert B The value of _exptl_crystal_size_max is > 1.0"

The crystal must be smaller than size of the incident beam at the crystal. For most diffractometers with fine-focus X-ray tubes and monochromated radiation, this size is unlikely to be much greater than 0.5 mm. Note that increasing the collimator diameter will not necessarily increase the size of the incident beam at the crystal. If the crystal used in the data collection is larger than the incident beam, then unpredictable errors in intensities will occur due to the varying amounts of the crystal in the beam for different reflection positions. If you obtain an alert about crystal size, then data should be collected with a smaller crystal. For the above reasons, it is recommended that crystals be generally smaller than 0.5 mm along any edge. If you used neutrons and obtained an alert about the crystal size, then you have forgotten to set _diffrn_radiation_type 'neutron'.