oskarener how to set & refine 2 atoms at special positions and fix interatomic distance of 2 atoms??? i try to do a refinement in topas, but it does NOT work. Cubic( 23.12227311) space_group Fd-3m, both atoms (B, N) should sit at 32e position (x x x). and B-N bonding at about 1.6A. below are i tried site N2 num_posns 32 x t1 0.12504` y t1 0.12504` z t1 0.12504` occ N 1.0000 beq 1 site B1 num_posns 48 x =t1+ 0.004 y =t1+ 0.004 z =t1+ 0.004 occ B 1.0000 beq 1 but topas not even output B1 position in cif file. then i tried site N2 num_posns 32 x t1 0.12512` y t1 0.12512` z t1 0.12512` occ N 1.0000 beq 1 site B1 num_posns 32 x t2 0.12437` y t2 0.12437` z t2 0.12437` occ B 1.0000 beq 1 Distance_Restrain(N2 B1 , 1.6, 0.03009`, 0.001, 10) but topas can not fix the bonding at 1.6+-0.001 any suggestion is welcome for this case. thanks.
ahhill Perhaps the following may help, prm N2_x 0.12504 site N2 num_posns 32 x = N2_x; y = N2_x; z = N2_x; occ N 1.0000 beq 1 Repeat for the second atom. Adrian