Placeholders for missing and inapplicable values

In addition to the numeric or character data values that a CIF data item may have, the special characters '?' and '.' may be used as data values irrespective of the data type assigned in a CIF dictionary. Note that this implies that a CIF reader must accept these special values for any data field, prior to type checking. Note also that they should appear in the CIF without surrounding quotation marks.

Both characters are used as placeholders, but with the following specific meanings.

Unknown value

The value '?' represents an unknown value of the quantity. It appears typically in template files to indicate data items whose value should be supplied by an application or user; or it may appear in the output from an application extracting information from a CIF in response to a request list.

Examples

Example 1: An extract from the Acta Cryst. C author's template (the full current version is available as ftp://ftp.iucr.org/pub/form.cif).
##############################################################################
###    CIF submission form for molecular structure report (Acta Cryst. C)  ###
##############################################################################

# This is an electronic "form" for submitting a structural paper to Acta Cryst.
#
# Note that all fields should be numeric or character type EXCEPT those which
# are flagged as 'text' - free-form text of any length may be included in
# these latter fields provided the text block begins and ends with a semicolon
# as the first character of a new line. Note also that the query marks
# '?' are significant as placeholders, and should not be deleted where a data
# item is not given, UNLESS the accompanying data name is also deleted.
# Lines should not exceed 80 characters in length.

#==============================================================================

# CHEMICAL DATA

_chemical_name_common                ?
_chemical_formula_moiety             ?
_chemical_formula_sum                ?
_chemical_formula_structural         ?
_chemical_formula_analytical         ?
_chemical_formula_iupac              ?
_chemical_formula_weight             ?
_chemical_melting_point              ?

#==============================================================================

# CRYSTAL DATA

_symmetry_cell_setting               ?
_symmetry_space_group_name_H-M       ?
_symmetry_space_group_name_Hall      ?

loop_
        _symmetry_equiv_pos_as_xyz   #<--must include 'x,y,z'
       ?

_cell_length_a                       ?
_cell_length_b                       ?
_cell_length_c                       ?
_cell_angle_alpha                    ?
_cell_angle_beta                     ?
_cell_angle_gamma                    ?
_cell_volume                         ?
Example 2: (a) is a portion of a defective CIF that does not list a complete set of unit-cell parameters. (b) is the output from a query intended to retrieve the unit-cell dimensions.
   _symmetry_cell_setting            monoclinic   
   _symmetry_space_group_name_H-M    'C 1 2 1'
   
   loop_ _symmetry_equiv_pos_as_xyz
                       x,y,z
                       1/2+x,1/2+y,z
                       -x,y,-z
                       1/2-x,1/2+y,-z
   _cell_length_a                     16.734(5)
   _cell_length_b                      7.830(4)
   _cell_length_c                     12.028(5)
   _cell_angle_beta                  119.12(3)
   _cell_volume                     1376.8(10)
(a) The generating application has omitted the orthogonal angles of a monoclinic cell.
   _symmetry_cell_setting                  monoclinic
   _symmetry_space_group_name_H-M          'C 1 2 1'
   
   loop_
   _symmetry_equiv_pos_as_xyz      
              x,y,z
      1/2+x,1/2+y,z
            -x,y,-z
     1/2-x,1/2+y,-z
   
   _cell_length_a                                    16.734(5)
   _cell_length_b                                     7.830(4)
   _cell_length_c                                    12.028(5)
   _cell_angle_alpha                      ?  # requested item not present
   _cell_angle_beta                                  119.12(3)
   _cell_angle_gamma                      ?  # requested item not present
   _cell_volume                                     1376.8(10)
(b) Output from the program QUASAR, given a request list designed to extract the cell parameters. Note how the QUASAR application annotates the missing values.

Inapplicable value

The value '.' represents an inapplicable value of the quantity. It may be inappropriate or meaningless (as in the case of _refine_ls_hydrogen_treatment reported for an inorganic salt containing no hydrogens). It may represent a value omitted intentionally, for some good reason. Or, in some circumstances (described in the _definition portion of the appropriate CIF dictionary entry), it may indicate the use of a default value.