**Dicvol06** is a well-known program for indexing powder diffraction
patterns (A. Boultif & D. Louer, "Program for the Automatic Indexing of Powder Diffraction Patterns
by the Successive Dichotomy Method", J. Appl. Cryst. **37**, 724-731 (2004).).

Dicvol can be used by Match! to derive unit cell parameters from the peak positions of marked experimental peaks. As an alternative to Dicvol, Treor can also be used for this purpose.

All that is required to run an indexing calculation are either experimental peaks or raw diffraction data. If only raw (profile) but no peak data are present when the indexing command is run, Match! will automatically execute the raw data processing before the actual indexing calculation is started.

Before running the "Indexing" command you should mark the peaks
to be taken into account in the indexing calculation.
If you do not mark any peaks, Match! will automatically use the 20 strongest peaks (if present) that
are **not yet covered by selected phases** and whose **relative intensity is larger than the
corresponding minimum value** (which can e.g. be adjusted using the red bar on the y-axis of the
diffraction pattern graphics) for indexing. You can also adjust the corresponding parameter “minimum
relative intensity for automatic peak usage” on the Indexing tab
or the parameter "minimum relative intensity for peak correlations" on the
Search-Match tab of the “Options” dialog.

You can run indexing either using the corresponding command from the "Tools" menu, or simply by pressing the corresponding button in the main toolbar. Depending on the current settings and situation, this will either bring up a dialog asking which indexing method (Treor or Dicvol) you would like to use, run the default indexing program, or display the table of indexing results that are already present (and from which also new calculations can be run).

Once you have run the indexing command and selected Dicvol as the indexing program to be used, the Dicvol parameter settings dialog will be displayed. The following parameters are available:

You can include or exclude **cubic**, **tetragonal**, **hexagonal**, **orthorhombic**,
**monoclinic** and/or **triclinic** cells. Normally, you should **start by excluding
monoclinic and triclinic** crystal systems.

In this section you can define **maximum values for the cell lengths**, **minimum and maximum
monoclinic cell angle**, as well as **minimum and maximum unit cell volume**.

You can define the **minimum figure-of-merit** as well as the **maximum number of unindexed
peaks** (in the Dicvol manual also called "impurity tolerance" N_IMP) for a solution to be accepted. Increasing the maximum number of unindexed peaks
will increase the probability to get a solution (unit cell), so this may be a way
out if you do not get a reasonable result with the standard settings. You should keep in mind though
that you have to check afterwards why the unindexed peaks are present in your pattern (maybe
they belong to an impurity phase or are just artifacts). **Note that by ignoring unindexed peaks it
is quite easy to get artificial unit cells that have nothing to do with reality!**

In addition, you can define the **max. peak position deviation** (in degrees 2theta), i.e. the
maximum 2theta difference between an experimental and its corresponding calculated peak. If the 2theta
difference is larger than this value, peaks are not regarded as being correlated (i.e. indexed).

If you know the arbitrary **formula weight and density** of your compound, and if the expected
number of
molecules or formula units in the unit cell is integer, you can enter the corresponding values
as well as the **max. density deviation** as additional criteria to restrict the cell volume and
hence the search space of the program. The value for the max. density deviation should be the maximum
expected density deviation plus about 5-10%. The choice of the value should also take the quality
of your diffraction data into account.

When the calculation has finished, a table of the unit cells (solutions) found by Dicvol will be displayed.
Please mark one or more solutions you would like to keep, then press <OK>.
You will then be taken to
the **indexing solutions** dialog where you can
evaluate the solutions (i.e. unit cells) that you have found up to now, inspect peak data, select
crystal system and space group, and finally **export** the solution or **add it as a new
manual entry** to the
match list (e.g. in order to proceed with
structure solution).

The **crystal system and space group suggested by Dicvol** are also copied to the individual
solution(s) in Match!. They can be seen in the corresponding dialog elements on the right-hand side
of the indexing results dialog if a corresponding
line is marked in the solution list at the top. You can of course modify these suggestions using
the corresponding dialog elements.

If you would like to take a look at the **original Dicvol output file**, you can do so
by marking the corresponding solution in the solution list and clicking the **View output** button
on the upper right-hand side.

- Indexing can only be successful if all peaks that are taken into account belong to a single phase!
- It is strongly recommended to check (and maybe correct) 2theta errors before trying indexing.
- Criterion for the best cell: Maximum figure-of-merit, minimum number of unindexed lines

The following hints on indexing have been taken from the Dicvol documentation:

- Be careful in using the impurity tolerance: spurious lines increases the risk to miss the correct solution!
- It is recommended to use a two- or three-stages procedure (i.e. triclinic lattices should
preferably be studied separately), for example:
- search in high symmetries down to orthorhombic: Line 2: n,itype,1,1,1,1,0,0
- search in monoclinic symmetry: Line 2: n,itype,0,0,0,0,1,0
- if necessary, search in triclinic symmetry: Line 2 : n,itype,0,0,0,0,0,1

- Trigonal symmetry case with rhombohedral lattice: the pattern is indexed with an hexagonal lattice, having a unit cell volume three times greater.
- Please, spend time to ensure the quality of your collected data. With accurate data, the success rate of Dicvol06 is very high. Peak positions should be extracted with a profile fitting software. An interactive program should be preferred, since automatic extractions can miss lines (low intensity, shoulder, ...).
- With bad data, the chance to obtain the correct solution is small and the calculation can be time-consuming.
- With modern X-ray powder diffractometers (the use of monochromatic radiation is recommended), absolute errors on peak positions lower than 0.02 degrees 2theta can be routinely obtained. For indexing purposes, errors should not (ideally) exceed 0.03 degrees in 2theta.
- With high resolution powder diffraction data (conventional or, particularly, synchrotron X-ray sources), the absolute error is usually less than 0.02 degrees (or even 0.01 degrees with ultra-high resolution) in 2theta; consequently, a maximum peak position deviation of 0.02 (or even 0.01) is recommended; the convergence of the dichotomy procedure will be improved. However, be sure that this condition is true for all lines used as input data. (Remember that all mathematical solutions within the input limits and error bounds are found, the greater they are the greater is the number of mathematical solutions).
- The maximum number of unindexed lines (also called "number of impurity lines", N_IMP) can be used in case of expected spurious lines (i.e. impurity lines, as well as observed lines out of the input error). N_IMP acts at all successive levels of the dichotomy algorithm. As soon as an indexing solution is retained, a least-squares refinement of lattice parameters is carried out. For this refinement a larger error on observed lines is considered. Then, a line rejected at the last dichotomy level can, by chance, be accepted with the refined lattice parameters.
- Note that the program Dicvol06 is executable from 7 lines- 8 lines if the 'zero-shift' is refined - (though it is not recommendable since LS refinement unstabilities can be expected).
- Long and short axis cases (dominant zone cases): if such cases are expected, the number N of lines used for searching the solution should, generally, be greater than 20.
- The minimum value for a linear lattice parameter has been fixed to 2.5 angstroms.
- Reliability of indexing solutions: read paragraph 8 of ref. 5 and refs 7 and 8.
- Note that with the option Dicvol04 (option =0), as soon as a solution is found, only solutions with smallest volumes will be subsequently retained. If (for some reasons!) you are not satisfied by the solution, you can run again the program with an input lower volume limit slightly greater than that of the found solution (the exhaustive search is then extended to a higher volume).
- Note that the search is exhaustive within the limits on the input data. In particular, the search is constrained by the higher and smaller bounds on parameters, volumes, selected FoM and absolute errors on peak positions. Please act on these parameters when using Dicvol06.
- A lattice metric singularity occurs when unit cells defining two lattices have an identical set of calculated d-spacings. This can be observed with high symmetry lattices, simple relations exist between the parameters of the two cells, as well as particular cell-volume ratios. A typical case is: an hexagonal cell [a, c, volume v] can be indexed with an orthorhombic cell [parameters: a/2, a sqrt(3), c, volume v/2]. Due to the strategy used in Dicvol, based on an analysis through decreasing symmetry, all cells should be, in principle, displayed in the output file (except if a solution is rejected by the input maximum volume).
- Possible space groups: look at the hkl conditions in the output list of the reviewing of the complete input data provided after a solution is found from the first N lines.