Andre Authier
Chair
IUCr Commission on Crystallographic Nomenclature

The undersigned are pleased to submit herewith the final report of the Working Group on a Crystalline Phase Identifier to the Commission on Crystallographic Nomenclature.
Signed:

	David Brown (chair)
     Sidney Abrahams (chair of CCN ex officio at time the
                  working group was established)
     +Michael Berndt (Crystal Impact)
     John Faber (International Center for Diffraction Data)
     Vicky Karen (National Institute for Science and Technology
                  and the Inorganic Crystal Structure Database)
     Sam Motherwell (Cambridge Crystallographic Data Centre)
     Pierre Villars (Pauling File)
     John Westbrook (Protein Databank)
     Brian McMahon (IUCr, consultant)

+ Deceased

*********************************************
REPORT OF THE WORKING GROUP ON CRYSTAL PHASE IDENTIFIERS
2005-3-21

SUMMARY
-------
The proposed crystalline phase identifier consists of a
number of components (layers) describing enough properties of the
phase to allow a unique identification.  These layers consist of
the chemical formula, a flag indicating the state of matter, the
space group number and the Wyckoff sequence.  They are defined in such a way that they can be incorporated into to the IUPAC-NIST Chemical Identifier (INChI) proposed by the International Union of Pure and Applied Chemistry (IUPAC). 

1. INTRODUCTION
---------------
The International Union of Pure and Applied Chemistry (IUPAC) has
been examining standards for the electronic representation of
chemical information, and as part of this effort it has
established a working group in conjunction with the USA National Institute for Science and Technology (NIST) to propose an IUPAC-NIST Chemical Identifier (INChI) which would uniquely identify any chemical compound appearing in an electronic database.  The INChI working group approached the Commission on Crystallographic Nomenclature(CCN) of the International Union of Crystallography (IUCr) to enquire if any conventions existed for a crystalline phase identifier that might be incorporated into INChI.  As the only such convention already approved by the CCN on phase transition nomenclature (Tolédano et al., 1998, 2001) is not readily adaptable for electronic use, the CCN established the Working Group on Crystalline Phase Identifiers to make recommendations that could be of use to the INChI working group. This document is the report of this Group.  

The Working Group carried out all its discussions by email,
initially independently of the INChI project, but the resulting
recommendations of the two groups developed structures for the identifiers that are so similar that incorporating the crystalline phase information into INChI should be trivial.  Our recommendations are therefore cast in the form of additions to INChI, in the knowledge that the resulting identifier can be as readily used by the crystallographic as by the chemical community.

2. TERMS OF REFERENCE OF THE WORKING GROUP 
------------------------------------------
The Working Group was charged with recommending to the IUCr Commission on Crystallographic Nomenclature:

1. The best method of defining a crystalline phase identifier
that uniquely and unambiguously identifies each crystalline phase
in a way that would allow it to be used to link the same material
appearing in different electronic databases.

2. The best way in which this identifier can be implemented, including its incorporation in the CCN recommended phase transition nomenclature.

Keeping in mind that the  primary purpose of the crystalline phase identifier is to allow the properties of a given material to be located in different databases, the Working Group should consult with appropriate crystallographic databases to ensure that the proposed identifier will be acceptable. 

3. MEMBERSHIP
-------------
The following were appointed members of the Working Group:
     David Brown (chair)
     Sidney Abrahams (chair of CCN ex officio at time the
                    working group was established)
     Michael Berndt (Crystal Impact) [deceased]
     John Faber (International Center for Diffraction Data, ICDD)
     Vicky Karen (National Institute for Science and Technology,
                    NIST and the Inorganic Crystal Structure
                    Database, ICSD)
     Sam Motherwell (Cambridge Crystallographic Data Centre,
                    CCDC)
	Jean-Claude Tolédano (Chair CCN Working Group on Phase
Transition Nomenclature) [resigned during the working phase of the Group]
     Pierre Villars (Pauling File)
     John Westbrook (Protein Databank)
     Brian McMahon (IUCr, consultant)

4. GENERAL CONSIDERATIONS
-------------------------
Early discussions revealed that many phases have not been
sufficiently well characterized to allow an unambiguous
assignment of an identifier, and in some cases phases have been
incorrectly characterized.  In these situations no identifier can
be expected to meet the requirements of the terms of reference 
but an identifier may be able to retrieve a number of possible
matches from which the user could make a final choice. 

For well-characterized materials the working group examined two
models.  In the first an arbitrary character string is assigned
to each crystallographic phase by a competent authority (similar
to the Chemical Abstracts Registry Number)  In the second a
character string is generated from the known properties of the
compound according to a defined set of rules.

The first choice was rejected on the grounds that we would be unlikely to find a competent authority willing to take on the
project.  Such an authority would require external funding, since
it would have to assign identifiers on request in a timely manner
and would have to maintain a public list of the identifiers
already assigned.

The second choice has the advantage that the identifier can be
constructed by anyone with access to the information needed to
characterize the material.  The identifier can be kept to a
manageable size because it only needs to include sufficient
information to distinguish between known phases.  Even if more
information about the phase is available, it is not included in
the identifier if it is not needed for characterization.  For
example, OsI3, which is known in only one crystalline form, is
fully characterized by its chemical formula alone and no further
information, chemical or crystallographic, need be included.

The first component of any phase identifier must be the composition and, where necessary, the isomer.  Only then does it
make sense to identify the crystalline form.  Since INChI is
designed to identify the composition and bond topology of the
compound, the Working Group's job was to suggest an identifier
that would distinguish between the different crystalline forms of
a given chemical compound.

5. THE INChI IDENTIFIER
-----------------------
Before presenting the recommendations of the Working Group, we
give a brief description of the proposed INChI which has, however, not yet been officially adopted by IUPAC.

The INChI working group is recommending an identifier made up of
several components or layers:  

The first (top) layer, which is always present, gives the
chemical composition.  The lower layers, which constitute the
identifier proper, are included only if they are necessary to
distinguish between two compounds with the same composition.  

The second layer distinguishes between different isomers by
describing the bond topology.  It contains several sublayers or
levels, the first giving the bonding topology ignoring all the
bonds to metals, cations and hydrogen atoms.  The second level
adds the bonding to fixed hydrogen atoms, the third adds the
bonding to variable hydrogen atoms (to distinguish between
tautomers if this is needed) and the fourth level adds bonds to
metal atoms and cations and is used in the rare cases when a
compound forms different coordination isomers.

The third layer contains information on chiral centers and is
included only when it is necessary to distinguish between
stereoisomers.  

The fourth layer is used to identify isotopically enriched
compounds.  Further layers can be added as needed.  For many
compounds only the first layer is needed and for most of the
others it is only necessary to add the top levels of the second
layer.

5.1 Construction of the IUPAC chemical identifier
------------------------------------------------- 
At the time of writing the final form of INChI had not been 
fixed, but sufficient details had been developed to allow the
definition of a crystalline phase identifier.  INChI can be formatted in different ways, in particular as an ASCII string or an XML file.  As the ASCII string is more compact and easier to follow, INChI is described below in this format.  In the following example a slash, /, is used to separate the layers. 

1.00Beta/C6H9N3O3/CT:7-4(10)1-2(5(8)11)3(1)6(9)12/H:1-
3H,(H2,7,10)(H2,8,11)(H2,9,12)/SC:1-,2-,3-
/I:(1D)/SC:m/is:0/ST:abs

The following is an explanation of the above INChI.  The important items are the first three or four - the remainder in this example deal with a description of the stereochemistry and will not be frequently used.  

1.00Beta/                           # Version of INChI
C6H9N3O3/                           # Sum formula
# The identifier proper begins here
CT:7-4(10)1-2(5(8)11)3(1)6(9)12/    # Basic connectivity
H:1-3H,(H2,7,10)(H2,8,11)(H2,9,12)/ # Hydrogen connectivity
SC:1-,2-,3-/                        # Stereocenters, sp3
I:(1D)/                             # Isotopes (H1 is deuterium)
SC:m/                               # Same as in main
is:0/                               # Inverted stereo (absolute
                                      stereo only)
ST:abs                              # Abs (absolute), rel
                                      (relative) or rac (racemic)
# End of identifier

The identifier may be followed by auxiliary information such as
atomic coordinates.  These are not part of the identifier proper
and are not shown here.  They are not further discussed in this report.

All but the first two items (which are required and are not part
of the identifier proper) are introduced by one of the following
tags:

"CT:";  /* connectivity */
"H:";   /* H-atoms */
"C:";   /* charge */
"DB:";  /* double bond stereo */
"SC:";  /* stereo centers sp3 */
"is:";  /* mark sp3 inverted stereo */
"SR:";  /* mark sp3 racemic stereo */
"ST:";  /* abs, rel, rac */
"I:";   /* isotopic atoms */
"fH:";  /* fixed H -- first item in non-taut */
"N:";   /* orig. at numbers in canonical order */
"NT:";  /* non-tautomeric orig. at numbers */
        /* in canonical order -- first item */
        /* in non-tautomeric aux info */
"E:";   /* atoms equivalence */
"tE:";  /* tautomeric groups equivalence */
"iC:";  /* inverted (stereo) Centers */
"iN:";  /* inverted sp3 stereo orig. atom */
        /*     numbers in canonical order */
"NI:";  /* isotopic orig. at numbers in */
        /* canonical order */
        /* first item in isotopic aux info */
"TR:";  /* transposition of components in */
        /* non-tautomeric representation */
"CRV:"; /* charges, radical, valence*/
"XYZ:"; /* xyz-coordinates */

The development of INChI has so far focused on organic molecules and resolving isomers, tautomers and enantiomers.  Version 1.0 will therefore be restricted to describing the topology of finite
molecules and is not being designed to describe the connectivity
of infinite structures.  This should not present a problem for
devising an INChI for crystalline phase identification, because if the composition and space group of an infinitely connected
inorganic compound are given (the two essential layers for any
phase identification), the connectivity is rarely needed.

6. RECOMMENDATIONS
------------------
We RECOMMEND that the phase identifier be included as part of the proposed INChI symbol and that the crystallographic characterization appear in INChI in three additional layers, which are numbered 5, 6 and 7 in the recommendations below.

Note that it is only necessary to define the format of the value of each layer.  The layers can be assembled in various ways as they are in the INChI standard, e.g., as an ASCII string labelled with tags (used in this document) or as an XML file.  There is no canonical form for the whole identifier, only for the individual layers which may be used with or without their associated tag. 

6.1 Layer 5: State of Matter
----------------------------
This layer gives the state of matter: gas, liquid, crystal etc.
according to the following enumeration list:
            gas         gas phase
            liq         liquid phase
            sol         solid phase of unknown form
            xtl         crystalline solid
            qxl         quasi-crystal
            ams         amorphous solid
            lxl         liquid crystal or other anomalous
                                quasi-liquid phase

Only if this flag is set to xtl would layers 6 and 7 be needed.  A crystal is defined as a phase for which, in principle, it is possible to assign a space group, even if that space group is only related to the full space group (as in the case of aperiodic structures).   The presence therefore of a space group field implies that the state of matter is xtl.  In the case where the space group is given the state of matter field is redundant and may be omitted (though it is included in some of the examples below by way of illustration).

6.2 Layer 6: Space Group
------------------------
This layer contains the space group number as given in
International Tables for Crystallography Vol. A.  It consists of
a number between 1 and 230 that uniquely identifies the space
group type except for aperiodic crystals, cf. Jansen et al.(2002).  The only ambiguity occurs for space groups such as
P41 (76) and P43 (78) that are identical except for their
chirality which is more appropriately identified in the INChI
stereochemistry layer.  Chirality is an important molecular
property but the chirality of a crystal, which is often not
determined, is usually only of interest if the crystal contains a
chiral molecule.  Chiral space groups should therefore be treated
as equivalent.  We recommend that only the lowest number of each
chiral pair of space groups be given, but search algorithms
should equivalence these pairs in case the higher space group
number is inadvertently used.  The equivalent space groups are listed below.

Problems in assigning the space group can arise in several
situations.  Many inorganic compounds have polymorphs with
similar structures that crystallize with different, but related,
space groups.  In these cases it is easy to assign the wrong
space group if only a subcell of the true crystallographic unit
cell is reported.  Incommensurately modulated structures, which are frequently associated with polymorphism, have additional
symmetries that do not appear in the standard table of space
groups.  Usually an average space group can be assigned, but
this is not always unique. Quasicrystals cannot be assigned a
conventional space group and are best treated as a different state of matter (see Section 6.1 above). 

6.3 Layer 7. Wyckoff Sequence
-----------------------------
In the rare event that two phases of the same compound have the
same space group, the Wyckoff sequence can be given.  This is a
list containing the Wyckoff letters associated with the occupied
special positions (sites of high symmetry).  Details of the
special positions and their Wyckoff letters for all 230 space
groups are given in International Tables for Crystallography Vol.
A.  Each letter is accompanied by a number indicating the number
of symmetry-independent atoms occupying sites of that kind (the
default number is 1), e.g, 'a1 d1 i6' which is written as 'adi6'.  The enumeration list contains all the letters of the alphabet plus & for 'alpha' and the letters are listed in alphabetic order.  Before determining the Wyckoff sequence it is essential that the structure be standardized using the procedure described in Section 7.2.4.

6.4 Possible further layers.
----------------------------
There are a few cases where the layers proposed above do not fully differentiate between distinct phases of the same compound.  For example metallic iron (see Section 8.2 below) has two body centered cubic phases separated by a face centered cubic phase.  These two phases have exactly the same identifier using the layers defined above.  However, they could be differentiated using their reduced cells.  With experience in using the layers defined above the need for further layers may become apparent.  At that time it would be appropriate to consider what further layers should be added.  Possibilities include the reduced cell, an incommensuration flag or an indicator of magnetic or electric properties.

7. PROPOSED ADDITIONS TO INChI FOR PHASE IDENTIFICATION
------------------------------------------------------
This section provides the text that should be inserted into any INChI definition that incorporates the proposals of this document.

7.1 New tags
------------
The following is a list of additional tags required for phase
identification expressed in the form of an INChI.  These would be
used in conjunction with existing INChI tags:

"PH:"  /* phase or state of matter. Allowed values are: */
       /* gas, liq, ams, sol, xtl, lxl, qxl */
"SG:"  /* Space group number, integers between 1 and 230 */
"WS:"  /* Wyckoff sequence, any lower case letter */
       /* or & (for alpha), possibly separated by numbers */

7.2 Formal definitions
----------------------
7.2.1 Composition
---------------
The composition layer in INChI for a crystalline phase must give
the contents of the formula unit of the crystal.  This is a unit
in general no smaller than the crystallographic asymmetric unit
and no larger than the primitive unit cell.  It is NOT the same
as the formula of the molecule of interest unless the molecule is
the only component of the crystal.  All components, including
solvents of crystallization, MUST BE EXPLICITLY INCLUDED. 
Wherever possible the formula unit is chosen so that the
multipliers of the elements are integers with no common divisor. 
In cases where it is not possible to choose a formula unit smaller than the primitive unit cell without using non-integral multipliers, e.g., FeS1.83 = Fe1.09S2, La1.95NiO4.31 and many minerals, the size of the formula unit is indeterminate and only the relative multipliers are meaningful.  Testing should be carried out in this case by normalizing the multipliers.  Any normalization can be used but an obvious one would be to reduce the largest multiplier to 1.00 and the others in proportion.  When non-integral multipliers are encountered, searches should include a tolerance factor to allow for experimental uncertainties or to retrieve related compounds of the same phase having a similar but not identical composition.  The tolerance should be large enough to recognize that Phase Identifiers that include trace elements are equivalent to Phase Identifiers in which the trace elements have been omitted either because they were not determined or because they were not considered to be important. The size of the tolerance factor is not defined in this standard and its choice will be determined by the nature of the required search.  For example, a search on FeS2 might include a tolerance factor of 0.2 to be sure of locating all examples of the phase.

7.2.2 PH:  The state of matter  
------------------------------
Seven flags are defined for a number of different states.

          gas 
          liq  liquid
          ams  amorphous  
          sol  solid of unknown form 
          xtl  crystal (capable of being assigned a space group)
          lxl  liquid crystal 
          qxl  quasi-crystal 

Only if the value of PH is 'xtl' will the following two layers be
meaningful.  Therefore if the SG: field is given, the PH: field may be omitted.

7.2.3 SG:  Space group
--------------------
This is a number between 1 and 230 inclusive, being the number of
the space group of the crystal as given in International Tables
for Crystallography Vol A.  The following space group pairs are
identical except for their chirality: 76=78, 91=95, 92=96,
144=145, 152=153, 169=170, 171=172, 178=179, 180=181, 212=213. 
Only the lower space group number of each pair should be used. 
The chirality is often not determined and is only significant if
the crystal contains a molecule whose chirality is described
elsewhere in the INChI.  However, one of the forbidden space group numbers may be inadvertently used and software should be prepared to convert it to its legal equivalent.  There are many cases where the true space group is not known.  Different approximate space groups might be assigned by different workers in which case a valid match would be missed, but there is little that can be done to overcome this problem.  Incommensurate phases should be assigned the space group of their parent structure (the first portion of the incommensurate space group symbol).

7.2.4 WS:  The Wyckoff sequence 
-----------------------------
This is an alphabetic list of the Wyckoff symbols (letters) of
the occupied special positions. International Tables for Crystallography Vol. A lists the Wyckoff letters for all
special position, that is, all sites having a crystallographically distinct site symmetry.  Each letter is followed by a number indicating the number of symmetry-independent atoms occupying sites of that kind (the
number 1 is omitted), e.g, 'a1 d1 i6' which is written as 'adi6'.  The enumeration list contains all the lower-case letters of the alphabet plus '&' for 'alpha' found in space group 47.  The letters are listed in alphabetic order but before determining the Wyckoff sequence, the structure must be normalized according to the algorithm described by Parthé & Gelato (1984, 1985).  This algorithm is based on the use of the standard space group settings given in International Tables A with the lattice parameters that are not determined by symmetry chosen according to a particular set of rules.  The atomic coordinates are then transformed to ensure a consistent choice of Wyckoff letter in the case where several Wyckoff letters refer to sites having the same symmetry*.  

------------------- Footnote --------------------------
The Parthé-Gelato (1984,1985) algorithm was selected because it is, to our knowledge, the only one that normalizes the whole structure including the atomic coordinates.  Other algorithms normalize only the space group setting and the unit cell basis, which by themselves are not sufficient to ensure a unique Wyckoff sequence.  This algorithm has been tested in several applications and has been shown to be robust and furthermore it has been implemented in the program STRUCTURE TIDY which includes the Wyckoff sequence as part of its output (Gelato & Parthé 1987).  The conventions used in the Parthé-Gelato algorithm differ from those adopted elsewhere in crystallography (see the discussion in Parthé & Gelato ,1984) since they are optimized to produce a robust structure normalization, while other conventions are designed for different purposes, e.g., to emphasize the physical or chemical properties of the crystal or to provide a cell descriptor that is easy to search.  The space group setting and unit cell basis used in the Parthé-Gelato algorithm, while essential for generating a unique Wyckoff sequence, are discarded after use and do not appear explicitly in the Crystalline Phase Identifier.  They therefore do not conflict with the different space group and unit cell conventions adopted by the databases using the Identifier. 
-------------------- End of Footnote --------------------

7.3 Examples
------------
In both the following examples of INChIs that define crystalline phases, the PH: field is not needed and the WS: field is also probably not necessary, but they are included for the purposes of illustration.
Further examples are given in Section 8.2

Rutile        1.02/TiO2/PH:xtl/SG:136/WS:af2

(CH3)3NCH2COOCaCl22H2O
              1.02/C4H15CaCl2NO4/PH:xtl/SG:33/WS:ae28

8. INCORPORATION OF THE PHASE IDENTIFIER INTO THE IUCr-CCN PHASE
TRANSITION SYMBOL 
------------------------------------------------------------
8.1 Description of the IUCr-CCN phase transition symbol.
--------------------------------------------------------
Recently the Commission on Crystallographic Nomenclature of the International Union of Crystallography adopted a Phase Transition Nomenclature (Tolédano et al. (1998, 2001)*.  

---------------- Footnote --------------------
* Available on the IUCr web page:  - under Commissions|Commission on Crystallographic Nomenclature|CNOM on-line information|Structural Phase Transition Nomenclature.  
-------------End of Footnote --------------------

The phase transitions are identified by the two phases that bracket the transition, so the nomenclature is more properly a nomenclature of the phases themselves.  The IUCr-CCN Phase Transition Symbol is composed of six fields defined as follows:
1. The common symbol used to identify the phase (e.g., alpha, II,
etc.).
2. The temperature range (pressure range or other external
condition) in which the phase is stable.
3. The Hermann-Mauguin symbol and number of the space group. 
More than one space group may be given, or the Bravais symbol may
be given if the space group is not known.
4. Z, the number of formula units in the conventional unit cell
(though the formula unit is not defined within the symbol).
5. The ferroic properties.
6. The structure type.

Any field may be omitted if inapplicable or if the value is not
known.  As the symbol was not designed for computer use, the formats are not tightly structured and may contain non-ASCII characters.  The intent of this symbol is to include the maximum identification information for the phase, whereas the philosophy of INChI is to include only the minimum information needed for phase identification.  Both include the space group but otherwise there is little overlap between them.  The two symbols are complementary and serve different purposes.  The INChI could be added to the CCN Phase Transition Nomenclature but it is not clear whether this would offer any advantage.  A method for combining the two, should this be desired, is described in Section 8.3.

8.2 Examples of the CCN Nomenclature and the INChI Crystalline Phase Identifier
-------------------------------------------------------
In each of the following examples the phase transition nomenclature is given first followed on the next line by the proposed INChI symbol. 

8.2.1 Three phases of potassium tellurium bromide (K2TeBr6) (Abrahams et al., 1984; Ihringer & Abrahams, 1984). 
----------------------------------------------------------

I|>434K|Fm(-3)m (225)|Z = 4|non-ferroic|Type = K2PtCl6. 

	1.02/Br6K2Te/SG:225

II|434-400K|P4/mnc (128)|Z = 2|ferroelastic|3 variants.

	1.02/Br2K2Te/SG:128

III|< 400K|P21/n (14)|Z = 2|ferroelastic|12 variants.

	1.02/Br6K2Te/SG:14

8.2.2 Iron (Fe) (Donohue, 1974)
-------------------------------

delta|1663K|Im(-3)m (229)|Z=2|non-ferroic|Type = W.Melting at
1808 K.

	1.02/Fe/SG:229

gamma|1663-1183K|Fm(-3)m (225)|Z = 4|non-ferroic|Type = Cu.

	1.02/Fe/SG:225

beta|1183-1043K|Im(-3)m (229)|Z = 2|paramagnetic|Type = W.

	1.02/Fe/SG:229

alpha|<1043K|Im(-3)m (229)*|Z = 2|ferromagnetic|Type = W. 
*Magnetic structure is pseudocubic.

	1.02/Fe/SG:229

epsilon|13GPa|P63/mmc (194)|Z = 2|-|Type = Mg.

	1.02/Fe/SG:194

Note that the delta and beta phases both have the body centred cubic structure and therefore have identical INChIs, but they are distinct phases with different cell dimensions.  The Wyckoff sequence /WS:a/ is omitted because it is the same for both phases and does not further distinguish between them.  The correct space group of untwinned alpha (ferromagnetic) iron cannot be isotropic Im-3m (229) but the deviation from higher symmetry is very small  and the correct space group is not yet determined.  This illustrates one of the problems that will be encountered in the use of this identifier.  At worst, searching on Fe/SG:229 will bring up a small number of false matches.

8.3 Examples of how the INChI might be incorporated into the CCN phase nomenclature 
---------------------------------------------------------------
The INChI can be included in the CCN Phase Nomenclature in several different ways.  We recommend that the INChI be placed in front of the phase nomenclature using vertical line separators rather than slashes.  A double vertical line separates the two identifiers.  If desired a program could easily split off the INChI for further analysis.  The first example is the same compound as shown in Section 8.2.1, the second example is the same compound as shown in Section 7.3.

1.02|Br6K2Te|SG:14||III|<400K|P21/n(14)|Z=2|Ferroelastic|12
variants

1.02|C4H15CaCl2NO4|SG:33|WS:ae28||XVI|<50K, 4GPa,<180K|
Pn21a(33)|Z=4|Ferroelectric|Nonmodulated ferroelectric
polarization along b

9. THE USE OF PHASE IDENTIFIER IN DATABASES
--------------------------------------------
Since the INChI Phase Identifier is parsable, each of the layers
can be formatted and stored in any way that suits the needs of a
particular database.  Most crystallographic databases will
already have fields containing the sum formula and the space
group number, and can readily add a field for the Wyckoff sequence if this is needed.  The 'state of matter' field, PH:, and the fields describing molecular properties, would require further fields in the database if they were present.  The canonical form of the INChI could be recreated at any time if required or it could be parsed into its respective layers.  It is not appropriate to carry out a search using the canonical form of INChI.  Searches must be carried out layer by layer since two different identifiers may not contain the same number of layers, or the search may not be carried out to its full depth if, for example, chirality or isotopic content were not important.

All the proposed layers can be searched by looking for identical
bit sequences, though the SG: field should be initially screened
for illegal numbers, and the composition field should be
normalized if non-integral multipliers are present.

REFERENCES

Abrahams, S. C., Ihringer, J., Marsh, P.  & Nassau, K.  (1984). J. Chem. Phys. 81, 2082-2087.

Donohue, J. (1974). The Structure of the Elements. New York: John Wiley & Sons.

Gelato, L.M., Parthé, E. (1987). J. Appl. Cryst. 20, 139-143.

Ihringer, J. & Abrahams, S. C. (1984) Phys. Rev.  B30, 6540-6548 (1984).

Janssen, T. , Birman, J. L., Dénoyer, F., Koptsik, V. A., Verger-Gaugry, J. L., Weigel, D., Yamamoto, A., Abrahams, S. C. & Kopsky, V.(2002). Acta Cryst., A58, 605-621.

Parthé, E., Gelato, L.M. (1984). Acta Cryst. A40, 169-183,

Parthé, E., Gelato, L.M. (1985). Acta Cryst. A41, 142-151.

Tolédano, J.-C., Glazer, A. M., Hahn, Th., Parthe, E.,  Roth, R. S. Berry, R. S., Metselaar, R. & Abrahams, S. C. (1998). Acta Cryst. (1998). A54, 1028-1033.

Tolédano, J.-C., Berry, R. S.,  Brown, P. J.,  Glazer, A. M.,  Metselaar, R., Pandey, D., Perez-Mato, J. M., Roth,  R. S. & Abrahams, S. C. (2001). Acta Cryst. (2001). A57, 614-626.

END OF DRAFT REPORT OF THE WORKING GROUP
---------------------------------------