SYMMS02

PROC-NAME: SYMMS02 Type_1

PURPOSE: To check that the _symmetry_cell_setting is consistent with the _cell_length_ and _cell_angle_ values.

PROCEDURE:

IF _symmetry_cell_setting == 'triclinic'
AND { _cell_length_a == _cell_length_b
OR _cell_length_a == _cell_length_c
OR any _cell_angle values == 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should not be equal for a triclinic cell"
   "Alert B The unit-cell lengths a and c should not be equal for a triclinic cell"
   "Alert B No unit-cell angle should be equal to 90 for a triclinic cell"

IF _symmetry_cell_setting == 'monoclinic'
AND { _cell_length_a == _cell_length_b
OR _cell_length_a == _cell_length_c
OR all _cell_angle_ values == 90.
OR two _cell_angle_ values != 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should not be equal for a monoclinic cell"
   "Alert B The unit-cell lengths a and c should not be equal for a monoclinic cell"
   "Alert B All angles should not be 90 for a monoclinic cell"
   "Alert B Two angles should be 90 for a monoclinic cell"

IF _symmetry_cell_setting == 'orthorhombic'
AND { _cell_length_a == _cell_length_b
OR _cell_length_a == _cell_length_c
OR all _cell_angle_ values != 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should not be equal for a orthorhombic cell"
   "Alert B The unit-cell lengths a and c should not be equal for a orthorhombic cell"
   "Alert B At least one angle should be 90 for an orthorhombic cell"

IF _symmetry_cell_setting == 'tetragonal'
AND { _cell_length_a != _cell_length_b
OR all _cell_angle_ values != 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should be equal for a tetragonal cell"
   "Alert B All angles should be 90 for a tetragonal cell"

IF _symmetry_cell_setting == 'rhombohedral'
AND { _cell_length_a != _cell_length_b
OR _cell_length_a != _cell_length_c
OR _cell_angle_alpha != _cell_length_beta
OR _cell_angle_alpha != _cell_length_gamma
OR _cell_angle_ values == 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should be equal for a rhombohedral cell"
   "Alert B The unit-cell lengths a and c should be equal for a rhombohedral cell"
   "Alert B Cell angles alpha and beta should be equal for a rhombohedral cell"
   "Alert B Cell angles alpha and gamma should be equal for a rhombohedral cell"
   "Alert B All angles should not be 90 for a rhombohedral cell"

IF _symmetry_cell_setting == 'trigonal' OR 'hexagonal'
AND { _cell_length_a != _cell_length_b
OR _cell_angle_alpha != 90.
OR _cell_angle_beta != 90.
OR _cell_angle_gamma != 120. } issue ALERT B
   "Alert B The unit-cell lengths a and b should be equal for a trigonal/hexagonal cell"
   "Alert B Cell angla alpha should be 90 for a trigonal/hexagonal cell"
   "Alert B Cell angla beta should be 90 for a trigonal/hexagonal cell"
   "Alert B Cell angle gamma should be 120 for a trigonal/hexagonal cell"

IF _symmetry_cell_setting == 'cubic'
AND { _cell_length_a != _cell_length_b
OR _cell_length_a != _cell_length_c
OR all _cell_angle_ values != 90. } issue ALERT B
   "Alert B The unit-cell lengths a and b should be equal for a cubic cell"
   "Alert B The unit-cell lengths a and c should be equal for a cubic cell"
   "Alert B All angles should be 90 for a cubic cell"

The unit cell parameters do not seem to be consistent with the expected constraints implied by the crystal system specified under _symmetry_cell_setting. Check that the crystal system has been correctly specified and that the unit cell parameters have been constrained where necessary.

For example, a monoclinic unit cell should not be specified with an angle of 89.98 deg. for an angle that should be exactly 90 deg due to the symmetry requirements. Such unit cell parameters should be constrained to appropriate values during the unit cell refinement. Similarly, a tetragonal unit cell should have identical values for the lengths of the a and b axes.

Of course, it is possible that two unit cell axes coincidentally have virtually equal lengths in a low symmetry crystal system or that a monoclinic structure has a beta angle that is indistinguishable from 90.0 deg. In such cases, these alerts can be ignored.

Full list of validation algorithms