how to apply external stress along one direction?

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Giblee · #1 Post by **Giblee** » Wed Feb 22, 2006 7:34 pm

If PSTRESS is used, the external pressure is applied uniformly to the cell, isn't it? How to add axial pressure or stress then?
thanks

#2 Post by **admin** » Fri Feb 24, 2006 1:12 pm

via setting PSTRESS only hydrostatic pressure can be applied, you are right. There is no automatic way to do axial pressure, you have to do a series of calculations with different cell volumes (and shapes) do model that

Giblee · #3 Post by **Giblee** » Fri Feb 24, 2006 6:37 pm

can I do an axial volume change, say, between -1.0% to 1.0% at a step of 0.01% along z axis? But when I relax the system at each stage, the volume should be fixed and only ions are allowed to move right?

Veronika · #4 Post by **Veronika** » Mon Feb 27, 2006 8:50 am

For this calculations you should have a look at:
http://cms.mpi.univie.ac.at/vasp/vasp/node103.html
and see how to fix select the proper ISIF.

Giblee · #5 Post by **Giblee** » Mon Feb 27, 2006 4:52 pm

When they systems is being relaxed at different values of the lattice constant along the z axis, (namely different volume) shall I allow shape chage at ? Or no shape change? I am going to calculate z axial strain energy, if I change the shape, will it still be a z axial strain? Thanks

Giblee · #6 Post by **Giblee** » Mon Feb 27, 2006 6:15 pm

When they systems is being relaxed at different values of the lattice constant along the z axis, (namely different volume) shall I allow shape chage at ? Or no shape change? I am going to calculate z axial strain energy, if I change the shape, will it still be a z axial strain? Thanks

#7 Post by **admin** » Wed Mar 01, 2006 10:52 am

This depends on the experiments you compare with:
1) the ions have to be relaxed.
2) V, shape:
As soon as either V or the shape are free to relax, they relax to the minimum force (energy) configuration of the respective (hydrostatic) pressure (V fixed) or adapt the equilibrium V (shape fixed) . If you allow for a shape change, all lattice parameters are adjusted (you cannot fix c and allow for a shape change automatically)