This is a general trial-and-error indexing
program for X-Ray diffraction powder patterns. (i.e. all symmetries included)
Trial and error program for indexing of unknown powder patterns cubic-tetragonal-hexagonal-orthorhombic-monoclinic-triclinic
Dominant zone test is added for the
Dominant zone test is added for the
Higher order lines among the first seven
lines (used in the base line sets) are automatically excluded from the trial
phase of the calculations.
If a monoclinic or triclinic solution
is found the program will end with a unit cell reduction followed by a conversion
of the reduced cell to a conventional cell according to the metric symmetry.
The reduction should be valid unless systematic extinctions are found in
the trial cell.
If a satisfactory solution is found,
only the condensed output file is needed. It contains all relevant information
and only one indexed list.
The general output list (that is normally
not needed, cf. 5) will only list trials where M20 (or Mxx if less lines
are available) is 6 or more and not more than 3 lines among the first 20
(or xx) lines are unindexed.
If the keyword VOL is given with a
negative sign all symmetries are tested until a final solution is found
(--if possible). This option should only be used on fast computers. It should
NOT be used on a PC (cf. 13 below).
An algoritm for successive reduction
of trial-cell volumes is used in monoclinic and triclinic tests if a negative
VOL is given. It is based on the input cell volume limit and the number
of trial cells found with IQ (see keyword IQ) or more than IQ indexable
It is strongly recommended to give
only the first (well checked and accurately measured) 25 lines in the diffraction
data list (See LINE SET TWO ).
It is expected that more than 95 per
cent of monoclinic and higher symmetry patterns and probably more than 50
per cent of triclinic patterns will be indexed presupposed the data quality
is high (i.e. average differences between calculated and observed diffraction
angles less than 0.02 deg. and also the weak lines included in the data).
The experience of triclinic patterns is limited however.
Obs. It is important to check cubic,
tetragonal and hexagonal solutions by a second run with KS=0 and THS=0 (See
key-word list.) Do not trust cubic, tetragonal or hexagonal solutions without
an orthorhombic test.
The reason for testing the symmetries
in correct order (from cubic to triclinic) and to START the orthrhombic,
monoclinic and triclinic tests with dominant zone tests is that by this
procedure false solutions are effectively avoided.
Vectorization of the subroutines ORTAL,
MAEG and COUNT is essential in order to reduce the computing times on a
CONVEX 210. For a normal run on CONVEX only the keywords
CHOICE=X, (see key-word list) and
should be given after the diffraction data list. Computing times of more
than 1 minute is rare for monoclinic or higher symmetries. Computing times
of more than 5 minutes for a triclinic pattern has not yet been found.
For a VAX computing times may be more than 50 times longer. The source
code for VAX is not exactly the same as for CONVEX.
The input format for LINE SET TWO
(see below) is changed in agreement with the output format of the diffraction
data file from the Guinier-Hagg film scanner program SCANPI (at Stockholm
University). The change is mainly of interest for output print of intensities.
The original Key-word instructions
given below are relevant as long as a positive VOL parameter is given.
If VOL is given a negative value (see
13 above) the following key-words are fixed: MONO=135 and MONOSET=7. Other
key-words may be used as in the description below.
On the output lists M-TEST= xx UNINDEXED
IN THE TEST= y usually means that xx is identical with M(20) and y is
the number of unindexed lines whithin the first 20 lines (i.e. used for
MERIT test). If less than 20 lines are available xx and y refer to the
number of lines used.