Dear All,

I am doing tensile test on 30x30x4 lattice constants gold sample at 0K. Boundary conditions are p p s.

I use box/relax sample to stretch/compress.

```
fix 1 all box/relax aniso 8000 8000 NULL.
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When the target pressures are of same sign, the system is reaching the
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specified pressure at the end of minimization.
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When i use target pressures of opposite sign, say 8000 -8000, the pressure
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after minimization is
becoming -3.8 in both directions.
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Am i correctly using the commands. Is there any other method to
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stretch/compress the sample for the above boundary
conditions at 0K.
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Could anyone help me
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Thanks
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```
Sivashankar
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I would ask Aidan about this - he has had success using these options

with applied stress. But he’s gone for a couple weeks.

Steve

Remember, any time you specify unequal normal stresses, you are specifying a

non-hydrostatic stress state, and so LAMMPS has to add a strain energy

contribution to the energy that it is minimizing. This strain energy depends

on the arbitrary reference state, which is taken to be the starting box

dimensions. The minimization procedure will converge to a stress state that

will equal the one you specified *only if* the final state equals the

reference state. So, you can only get the right answer when you start with

the right answer. How nice. The solution is to allow LAMMPS to reset the

reference state to the current state periodically using the nreset keyword.

Read about that in the fix box/relax doc page. Also, convergence can some

times be a problem, this stuff is not foolproof. So, try to solve easier

problems first, view all results critically, and build up to what you really

want to do.

Finally, I worry a little that you sample is non-periodic in z. I am not

sure what the consequences of this might be.

Aidan