deform
This keyword is used to deform the simulation box, which can be used to do tensile tests.
Syntax
The deform keyord requires 4 parameters:
deform <A_per_step> <deform_x> <deform_y> <deform_z>
Here, A_per_step specifies the speed of the increase of the box length, which is in units of Ångstrom/step.
For example, suppose the box length (in a given direction) in the beginning of a run is 100 Ångstrom and this parameter is \(10^{-5}\) Ångstrom/step, then a run with \(10^{6}\) steps will change the box length by 10%.
This gives a strain rate of \(10^{8}\) s \(^{-1}\) if the time step is 1 fs.
The second parameter deform_x can be 0 or 1, where 0 means do not deform the \(x\) direction and 1 means deform the \(x\) direction.
The last two parameters have similar meanings for the \(y\) and \(z\) directions.
Example
For uniaxial tensile test, one can first equilibrate the system and then deform the box:
# equilibration stage
ensemble npt_scr 300 300 100 0 0 0 100 100 100 1000
run 1000000
# production stage
ensemble npt_scr 300 300 100 0 0 0 100 100 100 1000
deform 0.00001 1 0 0
run 1000000
Caveats
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.
In the equilibration stage, it is also recommended to use the NPT ensemble to obtain the zero strain state before applying the deformation.
One must control the three pressure components independently when using this keyword.