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 ******************************************************************** Brian 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 ********************************************************************
