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--- bond_valence_restraints [2011/12/01 23:52] – Fixed some topas code formatting and added some extra comment markers to fix auto code colouring rowlesmr
+++ bond_valence_restraints [2025/09/19 15:18] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+====== Bond Valence Restraints ======
+It's often useful to restrain bond valence sums rather than distances and angles.  See e.g. Ivana Evans' user meeting notes from Bad Herrenalb or original literature on La2Mo2O9.
+You can do this in Topas using the atomic_interaction lines and associated penalties below.  In the example below 1.900 is the Rij value for W-O.  I've listed the specific sites that each atom bonds to in the structure, though you could use e.g. O*.
+This example was in P1 with Z=1.  If Z is not equal to one then you may find that the valence sum (reported at the end of the line) is wrong.  You'd then need to divide the value calculated by Z (e.g. divide by 2/4/8).  Be very careful about this!
+<code topas>
+'this is valence penalty for a W site
+'
+prm !val_weight 1.0 'this weights the valence penalties relative to other penalties
+atomic_interaction  W1_1_val = (Exp((1.900-R)/0.37));
+ai_radius 3.5 ai_sites_1  W1_1 ai_sites_2 "O3_1 O2_2 O1_1 O6_2 O3_2 O5_1" penalty = val_weight ( W1_1_val-6)^2;
+prm  W1_1_num =  W1_1_val;:6.09633`
+'this is valence penalty for an O site
+'
+atomic_interaction  O1_1_val = (Exp((1.900-R)/0.37));
+ai_radius 3.5 ai_sites_1  O1_1 ai_sites_2 "W2_3 W1_1" penalty = val_weight ( O1_1_val-2)^2;
+prm  O1_1_num =  O1_1_val;:2.02801`
+</code>