you also might circumvent the problem by substituting the angle with a distance restraint (Si-Si for Si-O-Si, etc).
best miguel On 25 Nov 2008 at 9:39, Brian H. Toby wrote: > > When setting up a bond angle soft constraint in GSAS, one needs to > input the atomic sequence numbers for the three atoms. However, if two > of the three atoms are the same and have the equivalent positions > (e.g., Si-O-Si), the two atoms have the same sequence number from the > atomic list. GSAS seems to only accept three different atomic sequence > numbers. What should be done? > > > The angle constraints (as opposed to the distance constraints) do not > allow symmetry to be used. Thus the atoms must be distinct and must be > adjacent in the asymmetric unit. There is no easy way to constrain an > angle that involves the same atom twice. > > This might work, however: I have considered the idea of putting dummy > atoms (with occupancy zero) into the asymmetric unit and then > constrain the dummy atom to refine along with the real atom (getting > the symmetry directions correct). Then one can use the dummy atom in > the constraint. This should work, but I have not tried it. > > Brian > ******************************************************************** B > ri an H. Toby, Ph.D. office: 630-252-5488 > Materials Characterization Group Leader, Advanced Photon Source 9700 > S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426 > Argonne National Laboratory secretary (Marija): 630-252-5453 > Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov > ******************************************************************** > > > > > -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
