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Constraints not followed in structural relaxation?

Asked by Jan Cuperus

Hi there,

I was trying to do a structural relaxation of an atom on a surface, where I set the constraints such that the slab is frozen and the single atom can move only in the z -direction.

For the adsorbed atom, the following can be read form the .out file;
Input coordinate: 0.0 0.0 27.0305....
Constraints info: Constraint v=[ 0.70711, 0.70711, 0.00000] (1): pos-dir
Output coordinate: 1.668... -1.668... 26.218...

To me, it seems like the constraints in the x and y directions have not worked, or is there some other fdf argument that I should have used? Using Siesta trunk-786 version.

Thanks!

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Nick Papior
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Best Nick Papior (nickpapior) said : 		#1

This looks correct. You are constraining one direction.

You have requested a constraint along the vector 1,1,0. Thus it will remove forces along the x-y diagonal.
If you want x and y to be constrained you have to create two constraints, one per direction.

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Jan Cuperus (jpcuperus) said : 		#2

Thanks Nick Papior, that solved my question.