I will wade in on this topic quickly.
Rigid body constraints can be used for two linked bodies pretty easily -- what one does is define the origin of each body as an atom that shared between each body (note that this atom needs to be present in the atom list twice, so adjust occupancies to total accordingly.) Then one can constrain the origin for each body to be the same and but still give each body independent orientations. This trick could be used for more complex situations where more than two bodies share a single atom, but off-hand, I don't see this helping in the case of a perovskite.
One other idea that could help is the following: you could define your perovskite blocks individually without any regard to linkages, but then use soft-constraints (really strong ones!) to keep the linking atoms "tied together." Again, you will need to define the linking atoms multiple times (so some many need to be dummy atoms or split occupancies), but this should work for any arbitrarily complex arrangement of bodies.
Brian
On Friday, November 12, 2004, at 10:50 AM, Sang-Heon Shim wrote:
I wonder if the rigid-body constraints of GSAS can be used for the polyhedra which are linked each other, such as octahedra in perovskite structured materials. Or is the application of the rigid-body constraints strictly restricted to isolated polyhedra in crystal structures?
