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*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,

(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.

(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.