TRCELL will always end up with a primitive triclinic unit cell.
To increase the symmetry of the selected and refined unit cell, you
can use the command lattice .. .. step ..,
for example to get a primitive orthorhombic cell:
fix all
fit orientation
fit beam
fit distance
lattice P orthorhombic step 10
The program will do 10 steps of refinement, adjusting the cell parameters
to force them to the symmetry of the selected lattice between each step.
It may sometimes be necessary to transform the unit cell to a different
setting first, before running lattice .. .. step ...
The program lepage is an excellent program for determining the transformation
matrix between the current primitive triclinic unit cell and a potential
unit cell with higher symmetry. The transformation matrix can be given in
LAUECELL in the following way:
transformation 1.0 0.0 0.0 0.0 0.0 1.0 0.0 1.0 0.0
Only right-handed cell settings are allowed: make sure that the determinant of the
transformation matrix is positive.
If lepage suggests a transformation to an A, B, C, I or F Bravais lattice,
you can use the following procedure to obtain the correct lattice:
lattice P triclinic
fix all
fit orientation
fit beam
fit distance
transformation 1.0 0.0 -1.0 1.0 0.0 1.0 -1.0 -2.0 -1.0
lattice F triclinic
run INDXPAT
...
Intermezzo - we realize that the lattice system F triclinic does not exist. It
is just used in order to be able to simulate the old diffraction pattern using the new
cell parameters that still have to be adjusted, but with using the correct extinction
criteria. -
If the match between the predicted and observed pattern is still O.K.,
the unit cell was transformed successfully.