Volume 2 Chapter 10 Search Menu *nCELL

Function

These search keywords are used to match a set of unit cell parameters against the reduced unit cell parameters in the CSD.

Graphics QUEST3D Procedure

The equivalent test construction in Graphics QUEST3D is discussed later in this chapter under REDUCED-CELLS.

Basic QUEST Procedure

Suppose you have a set of unit cell dimensions and you wish to find all entries in the CSD which have the same unit cell dimensions (within a specified tolerance) then QUEST will allow you to execute this matching process.

Any hits will correspond to the same compound or isomorphous structures.

QUEST first transforms the input set of unit cell parameters to the Niggli reduced cell parameters and compares the latter against the reduced cell parameters stored in the CSD.

Ex.

T1 *PCEL MONO 11.43 6.59 11.39 90 95.68 90 0.05

PCEL indicates that the input cell is primitive

MONO indicates the monoclinic system

The six unit cell parameters then follow

Finally a tolerance is declared, in this case 0.05 Ångström.

(1) n in *nCEL takes the values :

P: primitive cell
A: A-centred cell
B: B-centred cell
C: C-centred cell
F: face-centred cell
I: body-centred cell
R: rhombohedral cell

(2) The crystal system abbreviations are :

TRIC: triclinic/anorthic
MONO: monoclinic
ORTH: orthorhombic
TETR: tetragonal
HEXA: hexagonal/trigonal
CUBI: cubic
RHOM: rhombohedral

(3) Although the Niggli reduced cell is unique for a given set of cell dimensions, under certain special circumstances it can be very sensitive to small changes in one or more of the cell parameters. Only the cell angles are subject to an abrupt change (Andrews et al., Acta Crystallogr. A36,248,1980) and for this reason *nCELL compares only cell lengths. A hit is recorded if the three reduced cell lengths agree within plus or minus the tolerance.

(4) In every case all six unit cell dimensions must be input explicitly even though some may be redundant (defined by the symmetry of the crystal system).

(5) If the tolerance value is omitted then a default value of 0.001 Ångström is used.

(6) For each *nCELL test the program outputs, for the Niggli reduced cell,

all six cell parameters
the Bravais lattice
the metric class
the volume

(7) Standard settings are assumed ie.

MONO: b unique
TETR: c unique
HEXA: c unique
RHOM: (hexagonal axes) c unique

Nevertheless, alternative settings are provided for monoclinic and rhombohedral.

(i) Monoclinic
The symmetry axis may be a or b(default) or c.
To define either a unique or c unique the appropriate cell length is set negative.
The two tests
```
T1 *CCEL    MONO   14.25    6.90   17.69   90   97.19  90
T2 *ACEL    MONO   17.69   14.25   -6.90   90   90     97.19
```
are equivalent; both define the same crystal lattice.
(ii) Rhombohedral
Either rhombohedral axes ( triad = [111] ) or hexagonal axes ( triad = c = [001] ) may be used.
The program will automatically assign the appropriate setting after examining the cell dimensions.
In either setting RHOM is redundant and may be omitted.
The two tests
```
T1 *RCEL   RHOM    12.345   12.345   15.678   90      90     120
T2 *RCEL   8.838    8.838    8.838    88.60   88.60   88.60
```
are equivalent; both define the same crystal lattice.

(8) Combined formula/cell searches can provide a powerful means of determining whether an unknown substance matches a database entry.

Consider an unidentified substance of formula C10 H7-9 O4

Preliminary X-ray measurements indicate a primitive orthorhombic lattice with approximate cell dimensions a=11.65, b=13.86, c=11.07Å.

T1 *PCEL ORTH 11.65 13.86 11.07 90 90 90 0.5
T2 *RESI C10 + H7-9 + O4
QUES T1 .AND. T2

Note the very large tolerance used since the cell dimensions are approximate values.

Related Keywords

*RCPn, *RCVOLUME, *METRICCLASS

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Volume 2 Chapter 10 Search Menu NOJOURNAL .