Only the fractional atomic coordinates
and the scale parameter are refined.
Typically the number of sites can be
guessed to some accuracy from the volume of the unit cell. However
the possibility of atoms residing on special positions complicates
matters. To cater for special positions the occ_merge keywords have
been included so that atoms of the same species approaching each
other have their occupancies reduced (Favre-Nicolin, V. and Cerny,
R. “Fox: Modular Approach to Crystal Structure Determination from
Powder Diffraction", EPDIC 8 proceedings, 2002.) , search the Technical
Reference for occ_merge.
Save solution.inp by clicking on
,
Alt-Tab to TA and start the structure solution. The animated fitting
updates the scan window and the OpenGL window both of which consumes
a fair amount of CPU time. These graphics can be turned off by clicking
on the icon
in the
Fit Window toolbar; this speeds up the structure solution process.
Also consuming CPU time is the Rwp plot of the Fit Window; it can
be turned off by displaying only "Text" in the fit window which
is accomplished by clicking on the left most icon of the Fit Window
toolbar.
The simulated annealing process continues
until the maximum number of iterations as defined by the keyword
iters is reached, if undefined then iters defaults to 500000. Typically
the User stops the simulated annealing process by clicking on the
green arrow of the Fit Window.
Finding the correct solution takes
less than 20 seconds on a 3GHz computer. To speed up the process
the Decompose(0.005) macro, found under the XInsert node of "Structure
Solution"), can be inserted at the scan level. The Decompose macro
contains the keyword yobs_to_xo_posn_yobs, search for it in the
Technical Reference.
The TA screen during structure solution
and with Decompose looks like the following:
When the structure solution process is
terminated by clicking on the green arrow the parameters that produced
the best solution is saved and the OpenGL window and the calculated
pattern is updated to reflect the best solution.
Inspection of the OpenGL window reveals
which atoms lie on top of each other and thus special positions
and degenerate sites can be easily identified. Atoms of the same
site that lie on top of each other identifies a special position.
Atoms of different sites that lie on top of each other identifies
degenerate sites. In the case of this structure two oxygen sites
merge into one with a corresponding occupancy of around 0.5. Similar
deductions can be made by inspection of bond lengths as outputted
using the append_bond_lengths keyword.