[QE-users] Constraints linear geometry

[Julien Claudot]

Dear all, 

I'm currently running calculations with simple linear molecules (for example : hydrogen cyanide HCN) adsorbed on graphene+metal surface. 
Surprinsigly, I faced few issues concerning the relaxation of these structures. Sometimes the molecules can not relax without forming an angle around 140° instead of 180° as expected. 

In order to fix this I used the CONSTRAINTs tag as shown here : 

CONSTRAINTS 
1 1.5 
'planar_angle' 33 34 35 


The 33, 34 and 35 atoms being the concerned atom. Even with the constraints it is sometimes impossible to obtain a logical relaxed structures with an angle around 180°. 
And sometimes the calculation are even crashing with the message " g = 0 is not satified ". 
I have read the doc about constraints of pw.x but nothing indicates me the way to resolve this. 
Also I would like to understang how the parameter CONSTR_TOL (equal to 1.5 in the example) affects the calculations (nothing in the doc about this point) ? 

Thanks 
-- 
Julien Claudot 
Post-doc 
IGCM UMR5253 
DAMP team 
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