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

International Tables for Crystallography (2006). Vol. G, ch. 3.1, pp. 76-79

Section 3.1.5. Constructing a DDL1 dictionary

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: bm@iucr.org

3.1.5. Constructing a DDL1 dictionary

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Dictionaries constructed according to DDL1 have quite a simple structure. The structure is summarized in this section; Sections 3.1.5.1[link]–3.1.5.4 provide more detail. Each definition is encapsulated within its own data block. Fig. 3.1.5.1[link] outlines the contents of the core CIF dictionary. The order of the data blocks has no significance, but it is common practice to start the file with the data block that describes the name, version and revision history of the dictionary itself and then to arrange data blocks in alphabetical order, sorted first on category then on names within a category. This practice is not always followed – for example, the powder dictionary is ordered by theme. The choice of order in a dictionary is only used for presentation and dictionary parsers should not assume or rely on any order of data blocks.

[Figure 3.1.5.1]

Figure 3.1.5.1 | top | pdf |

Schematic structure of core CIF dictionary. (a) Dictionary identifiers. (b) Definitions of categories and data items.

The name of a data block is usually constructed from the name of the data item it describes, e.g. data_refln_phase_meas. Where the data block describes an entire category instead of a single data item, the category name is followed by matching square brackets, which may contain an alphabetic code representing the dictionary name if it is an extension to the core dictionary (e.g. data_refln_[], data_audit_link_[ms]). Where the data block defines several data names, the initial common portion of the names is used with a trailing underscore (e.g. data_refln_).

A preliminary data block, by convention labelled with the header string data_on_this_dictionary, contains the dictionary identification information and revision history. The name of the dictionary itself (given by the data name _dictionary_name) is conventionally of the form cif_identifier.dic, where the identifier is a short code for the topic area of the dictionary (e.g. `core' for the core dictionary, `pd' for the powder dictionary, `ms' for the modulated structures dictionary, `rho' for the electron density dictionary).

Data names are classified by category. The _category attribute is a character string intended to indicate the `natural grouping' of data items. If a data item occurs in a looped list, it must be grouped only with items from the same category. It is, however, permissible for a file to contain more than one looped list of the same category, provided that each loop has its own specific reference item identified by the _list_reference attribute of the data names included. Examples of this will be given below.

For each category, a data block is usually provided that contains information about the purpose of the category, generally illustrated with examples.

All other data blocks represent self-contained definitions of a single data item or a small set of closely related data items. The definition includes the physical units of and constraints on the values of the data labelled by the defined data name, and also information about relationships with other data items.

It is conventional, although not mandatory in DDL1 dictionaries, that the category name should appear as the leading component or components of a data name. For example, the data name _exptl_crystal_colour is a member of the core category EXPTL, while _exptl_crystal_density_meas is a member of the category EXPTL_CRYSTAL and _exptl_crystal_face_perp_dist is a member of the category EXPTL_CRYSTAL_FACE. However, it will be seen that there is no sure way of working out the category from the complete data name except by referring to its _category attribute in the associated dictionary. This differs from the DDL2 convention of including an explicit separator (a full stop) between the category name and the remainder of a data name.

While it is not mandatory that a data name should incorporate its category name as a leading component, authors are strongly encouraged to adopt this convention. A small number of core data items that did not conform to this convention have been deprecated in later releases of the core dictionary. However, in the powder dictionary the convention has been broken so that one can present data sets separately or merge them together. In this dictionary, some data names beginning with the strings _pd_calc, _pd_meas and _pd_proc all belong formally to the category PD_DATA. This allows calculated data values to be tabulated with raw and processed measurements if this is useful.

One other case where a data name does not begin with its associated category name is that of the pseudo data names such as _exptl_[] that appear in the dictionary to describe the purpose of a category (Section 3.1.5.3[link]). Such data names are always assigned the category CATEGORY_OVERVIEW and are further differentiated from other data names by having a data type of `null'.

3.1.5.1. The dictionary identification block

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As mentioned above, the dictionary file must contain information that unambiguously states its identity and version. In DDL1-based CIF dictionaries, this is achieved by itemizing the full set of dictionary attributes (see Section 2.5.6.5[link] ) within a data block named data_on_this_dictionary, as in Example 3.1.5.1[link] from the core dictionary.

Example 3.1.5.1. A DDL1 dictionary identification block.

[Scheme scheme3]

3.1.5.2. Irreducible sets of data items

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In general, a dictionary data block defines a single data item. However, there are instances where several related data names are defined in the same data block. Sometimes this has been done for convenience, to produce a compact listing of similar data names that have common attributes and whose small differences in meaning can best be expressed by a single definition. Such groupings are discouraged, except where they represent components of a larger entity that has no sensible meaning in the absence of any of the components. For example, the data block data_refln_index_ defines the three data items _refln_index_h, _refln_index_k and _refln_index_l that represent the Miller indices of a reflection. All three indices must have a value in order to specify a reflection and so each has no meaning in isolation.

Note that there is no formal method of expressing this close relationship within DDL1 except by grouping the definitions in the same data block in this way. In DDL2 dictionaries, it is common to assign the components of an irreducible set to a specific subcategory.

3.1.5.3. Category descriptions

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As discussed above, categories in DDL1 are intended as `natural groupings' of data items. To document the purpose of a category within a dictionary, `pseudo' data names are used. All pseudo data names are assigned a _category attribute of category_overview and have an associated _type value of `null'. They are also named by convention as _category_name_[dictionary code], for example _pd_data_[pd] for the description of the PD_DATA category in the powder dictionary (indicated by the code `pd' in square brackets). For the core dictionary, dictionarycode is not given, resulting in names like _exptl_[] to describe the EXPTL category.

Example 3.1.5.2[link] is a slightly edited extract from the core dictionary showing how a data block for a category description is composed, including the presence of an example.

Example 3.1.5.2. A category description in a DDL1 dictionary.

[Scheme scheme4]

Note that the dictionarycode extension allows a dictionary to include comments on items that it defines in a category already established in the core dictionary. For example, the modulated structures dictionary includes the category overview item _audit_link_[ms]. This describes the convention adopted to express the relationship between data blocks in a modulated structures data file using the _audit_link_ data names already defined in the core dictionary.

3.1.5.4. Data-item definitions

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The data blocks described in Sections 3.1.5.1[link] and 3.1.5.3[link] are used to identify the dictionary and to describe the nature and purpose of a category. The remaining data blocks in a dictionary provide the attributes of data values in a form suitable for machine extraction and validation. The following examples show how this is done for various types of data.

3.1.5.4.1. Definitions of single quantities

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Example 3.1.5.3[link] is the core dictionary definition of the data name for the ambient temperature during the experiment. Because this is a single (non-looped) value, the relevant data name is one among several discrete items in the DIFFRN category. No further description of its relationship to other data items is required.

Example 3.1.5.3. A simple definition of a data item describing a physical quantity.

[Scheme scheme5]

The type of the associated data value (numb for numerical) is specified, together with any constraint on its legal value. The range specified ( 0.0:) indicates that it may be any non-negative real number. The physical units of the quantity are also indicated.

The _definition attribute is a concise human-readable documentation of the meaning associated with the data name.

Example 3.1.5.4[link] is taken from the powder dictionary and illustrates a data item that can have only one of a limited set of values. This data item indicates the geometry of the experiment. The associated data value is of type char and may legally take only one of the two possible values listed.

Example 3.1.5.4. A data item that can take only one of a discrete set of allowed values.

[Scheme scheme7]

3.1.5.4.2. Looped data

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Many of the attributes of looped data items, such as their physical units or valid numerical values, may be defined in exactly the same way as for non-looped data. However, more care needs to be taken to describe the relationships between different looped data items.

Consider the following example listing of some three-dimensional atom-site coordinates and displacement parameters. [Scheme scheme6]

These loops, or tables of values, are properties of atom sites, each identified by a label such as O1. The definition of a data name such as _atom_site_U_iso_or_equiv expresses this by using the DDL1 _list_reference attribute (Example 3.1.5.5[link]).

Example 3.1.5.5. Definition relating a looped data item to the item used to identify a `loop packet', or row of entries in a table.

[Scheme scheme8]

For an entry in the table to make sense, the site identifier must be present, so the definition for _atom_site_label declares it a mandatory item within its list (Example 3.1.5.6[link]).

Example 3.1.5.6. Definition of a mandatory item within a loop.

[Scheme scheme9]

It is common for an atom-site identifier to be used in several related tabulations in a particular crystal structure description, and in a CIF description this means that it may occur in several different looped lists. The dictionary definition gives a formal account of this by listing the data names in other looped lists which are just different manifestations of this same item. This is done using the _list_link_child attribute, which identifies the data names to which the one being currently defined is `parent'. In Example 3.1.5.6[link] (which is a subset of the full list in the core dictionary), _atom_site_aniso_label, _geom_bond_atom_site_label_1 and _geom_bond_atom_site_label_2 are identified as children of _atom_site_label.

It can been seen immediately that _atom_site_aniso_label is the atom-site identification label appearing in the second table in the example listing above, and the _geom_bond_ items are clearly atom-site labels in a table of bonding properties between specified sites. There is, however, a difference between the two secondary tables: the bond-properties table is described by data items in the GEOM_BOND category, but the table of anisotropic displacement parameters includes data names that have the same _category attribute as the coordinate data items, namely ATOM_SITE. The latter is an example of multiple lists or tables belonging to the same category, a feature permitted only in DDL1-based data files.

3.1.5.4.3. Units

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The physical units in which a quantitative data item must be expressed are identified by the DDL1 attributes _units and _units_detail. The latter is a character field describing the units; the _units attribute is a code that may be interpreted by machine. In DDL1-based dictionaries, type codes are purely conventional, and there is no mechanism for converting units or relating quantities in different units. Table 3.1.5.1[link] lists the units codes used in the DDL1-based dictionaries described in this volume. There can be some inconsistencies: two codes (`s' and `sec') are already in use to indicate the time unit of seconds.

Table 3.1.5.1| top | pdf |
Units codes and their interpretation in DDL1-based dictionaries

Unit code ( _units)Meaning ( _units_detail)
A Ångströms
A^-1^ Reciprocal ångströms
A^2^ Ångströms squared
A^3^ Ångströms cubed
Da Daltons
K Kelvins
Kmin^-1^ Kelvins/minute
Mgm^-3^ Megagrams per cubic metre
\ms Microseconds
deg Degrees
deg/min Degrees per minute
eV Electronvolts
e_A^-3^ Electrons per cubic ångström
fm Femtometres
kPa Kilopascals
kV Kilovolts
kW Kilowatts
mA Milliamperes
min Minutes
mm Millimetres
mm^-1^ Reciprocal millimetres
s Seconds
sec Seconds

The original CIF paper (Hall et al., 1991[link]) described a convention allowing physical quantities to be listed in a CIF in units other than those specified in the dictionary. Under this convention, a data name representing a value expressed in different units could be constructed by appending one of a series of known `units extension codes' to the standard data name. Thus _cell_length_a_pm would represent a cell length expressed in picometres instead of the default ångströms. This approach is now deprecated, and all quantities must be expressed in the single unit permitted in their definition block. However, to allow the formal validation of old CIFs, a `compatibility dictionary' is available which defines all data names that could have been constructed under this convention in a properly DDL1.4-compliant form. This dictionary should only be used for validating old CIFs, and must not be used to construct new data files. The dictionary is called cif_compat.dic in the IUCr CIF dictionary register (see Section 3.1.8.2[link]).

References

Hall, S. R., Allen, F. H. & Brown, I. D. (1991). The Crystallographic Information File (CIF): a new standard archive file for crystallography. Acta Cryst. A47, 655–685.








































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