Ralaxation of Au-molecule-Au junction

[huixiaodemao0712]

Hi，
I’m a newbie in ATK and get into trouble with the relaxation of the Au-molecule-Au junction.
(1)   What’s the difference between maximum force and maximum stress? I think I don’t understand their physical meanings.
(2)   I have built the molecular junction following Building a molecular junction: Au-DTB-Au. In the device optimization, I set the parameters:
maximum force=0.1, maximum stress=0.1, remove the tick from constraining the cell in the z-direction , and I didn’t constrain any atom.
However, in the relaxed device, I found some Au atoms in the surface layer depart from each other. Why did this happen? Is the optimization result reasonable and used in the next I-V calculation?

[zh]

1. Do you understand the concept (or physics meaning) of force and stress?
In the atomistic simulation, the terminology of 'force' usually means the force between atoms, while the "stress" is exerted on the cell. The former is correlated to the optimization of atomic positions and the latter for the length and shape of cell.

2. " I found some Au atoms in the surface layer depart from each other. Why did this happen?"
Why is it not possible to happen?!
It is better to make constraints on the atoms belonging to the electrode region and some of atoms in center region (at least those atoms just next to the electrode region)   

[huixiaodemao0712]

1. Do you understand the concept (or physics meaning) of force and stress?
In the atomistic simulation, the terminology of 'force' usually means the force between atoms, while the "stress" is exerted on the cell. The former is correlated to the optimization of atomic positions and the latter for the length and shape of cell.

2. " I found some Au atoms in the surface layer depart from each other. Why did this happen?"
Why is it not possible to happen?!
It is better to make constraints on the atoms belonging to the electrode region and some of atoms in center region (at least those atoms just next to the electrode region)  

Thanks a lot for your kind help!
1.I have learnt more about force and stress now according to your reply.
2.My molecular junction is consisted of electrode(3 layers),electrode extension(3 layers),surface(2 layers) and the molecule. You mean I'd better constrain the electrode and electrode extension layers, then relax the surface layers and molecule, am I right?

[zh]

Yes, you had better make constraint on the atoms in the electrode(3 layers) and electrode extension(3 layers).

[huixiaodemao0712]

Thank you Zh!

Kind regards.