# Tension over not uniaxial directions

Dear LAMMPS users,

I am working over a box simulating phase transformations, after obtaining the correct phases now I am doing tensile tests, my main reference is the material’s unitcell for the ‘hot’ phase so I use the lattice command and then replicate to achieve the desired size, the problem is that after the phase transformation I want to make some loading over the [101] direction, but I am not sure how to do that or which its the best way, the ideas I have came up with are,

1. I can export the atoms coordinates and then rotate them in Matlab but I would rather not to interrupt the process outside lammps.

2. Generate the atoms and define a box to obtain a flat face to apply the loading, then reorient the atoms so [101] gets aligned with [100] with the rotate in the displace_atoms command, then apply the loading as a uniaxial, is this possible and most of all correct?

3. I have tought also just to generate the atoms and then to define the box to obtain a flat face to apply the loading, but still I am not sure on how to apply a load in not uniaxial directions, would be fine to use the next fix deform for that? I guess combining it with NVT ensemble would be all right but what about NPT*, is it posible to use it as shown below?

*fix 1 all npt temp 300 300 1 xy 0 0 1 y 0 0 1 drag 1
fix 2 all deform 1 xz erate 0.001 units box remap x

As always thank you for your help and recommendations,

Mario Muralles

Your descriptions are very confusing, but it sounds like you would like to build a perfect crystal and deform it along [101]. This is not hard to do in LAMMPS, and there is more than one way to do it. However, if you just grab something that says “xz” and blindly hope that it gives you [101] strain you are guaranteed to fail. Here are two ways:

1. The most direct way is to build the crystal with the [101] direction oriented along x, y, or z. This can be done using the lattice command orient keywords and appropriate box dimensions. For a cubic lattice, this is very straightforward. For non-cubic it is a little more complicated to figure out the right orientation directions and box dimensions, but the idea is the same.

2. It is also possible to apply a biaxial strain that results in the correct tensile loading orientation. Specifying an “xz” strain in fix deform does not do this. You will need to do some pen-and-paper work, read the doc page for fix deform in its entirety, and carefully check your script to verify that it is doing what you think it is doing.

Aidan