# The deform keyword

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## Purpose

This keyword is used to deform the simulation box, which can be used to do tensile tests.

## Grammar

It requires 4 parameters:

deform strain_rate deform_x deform_y deform_z

Here, `strain rate`

specifies the speed of the increase of the box length, which is in units of A/step. For example, suppose the box length (in a given direction) in the beginning of a run is 100 A and this parameter is [math]10^{-5}[/math] A/step, then a run with [math]10^{6}[/math] steps will change the box length by 10%. This gives a strain rate of [math]10^{8}[/math] s[math]^{-1}[/math] if the time step is 1 fs. The second parameter `deform_x`

can be 0 or 1, where 0 means do not deform the [math]x[/math] direction and 1 means deform the [math]y[/math] direction. The last two parameters have similar meanings.

## Examples

### Example 1

- For uniaxial tensile test, one can first equilibrate the system and then deform the box:

# The equilibration stage ensemble npt_ber 300 300 0.1 0 0 0 0.0005 run 1000000 # The production stage ensemble npt_ber 300 300 0.1 0 0 0 0.0005 neighbor 1 deform 0.00001 1 0 0 run 1000000

- Currently, one must use the NPT ensemble when using this keyword. That is, the code assumes that the pressure components in the directions which are not deformed will be controlled.
- Also note that we have choose to update the neighbor list. When the system is deformed, the neighbor list might need to be updated.
- In the equilibration stage, it is also recommended to use the NPT ensemble to obtain the zero strain state before applying the deformation.

## Caveats

- See the example above.

## Related keywords

- None