Hi Siesta users,I need some help on the following problem.I made two new
compounds, one I solved crystal structure and the other not. However, based on
its synthesis, XRD, indexing, IR, and several physical characterization
methods, I may confirm that they are iso-structural. Of course, I can do
Rietveld refinement to get the crystal structure of the latter by using the
initial crystal structure of the first. The problem is the crystal structure I
solve involved 204 atoms in unit cell (or 68 atoms in a asymmetric unit) and
most of them is atom having atomic number less than 10. As you know, the lab
XRD or even synchrotron X-ray diffraction in this case is not much help. I
could not have chance to get neutron data as its schedule is full.The Rietveld
refinement diverged when I turn on the refinement of oxygen atoms. One of the
reasons is the initial structure is far from the true one. I definitely need to
somehow obtain the initial crystal structure for the refinement which i
s as close as the real structure.I intend to obtain it by using the crystal
structure relaxation in quantum espresso code or siesta code. As you know, I
could use some constrain to keep some of special positions of atoms fixed while
allowing the others move. However, for atoms related by the space group
symmetry, I want to utilize the advances. For example, atom A in Wyckoff
position (0,y,z) related to its mirror atom B (1/2,-y,z+1/2), whenever, the
code move A I would expect the code also move B in the way that keep this
relation. Based on my experience, the quantum espresso and siesta codes don't
have this implementation. One of the codes I know is lmto ultilize the space
group symmetry but don't allow structure relaxationSo I would ask the users in
the community some ideas or any code that allow me to do the structural
relaxation that keep the crystal space group. I would be very appreciated to
it.Best wishes,Sau Nguyen, U of H
