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====== 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>