# [lammps-users] About several parameters in fix deform command

Hi everyone,

I am confused when im doing unixial tension using the fix deform
command. I cannot understand the parameter N very clearly, although it
has been explained in the doc. Every Nth timestep during the run, the
simulation box is expanded, contracted, or tilted to ramped values
between the initial and final values. Does the 'ramped values'
generally depend on the total timesteps of the run?

For example,

fix 1 all deform 1000 z scale 1.3
run 10000
Will the 1.3 be divided 10 (10000/1000=10) times during the run?

For another example, use erate style

unit metal
...
timestep 0.02
fix 3 all deform 1000 z erate 0.00015
run 100000

1. What is the meaning of 1000 in the above fix deform command? Does
it mean a 0.00015 strain rate will applied to the box every 1000
timesteps? Can the 1000 be considered the box will be relaxed 1000
steps?

2. Does it have to do with the calculation of the engineering strain?
what is the strain after 100000 steps?

Regards,

Han

Steve

Hi everyone,

I am confused when im doing unixial tension using the fix deform
command. I cannot understand the parameter N very clearly, although it
has been explained in the doc. Every Nth timestep during the run, the
simulation box is expanded, contracted, or tilted to ramped values
between the initial and final values. Does the 'ramped values'
generally depend on the total timesteps of the run?

For example,

fix 1 all deform 1000 z scale 1.3
run 10000
Will the 1.3 be divided 10 (10000/1000=10) times during the run?

yes

For another example, use erate style

unit metal
...
timestep 0.02
fix 3 all deform 1000 z erate 0.00015
run 100000

1. What is the meaning of 1000 in the above fix deform command? Does
it mean a 0.00015 strain rate will applied to the box every 1000
timesteps? Can the 1000 be considered the box will be relaxed 1000
steps?

The 1000 means the same thing as above. A new box size (deformation)
will be created once every 1000 steps, at the specified erate.

2. Does it have to do with the calculation of the engineering strain?
what is the strain after 100000 steps?

I think the doc page describes this.

For all of these, I suggest you create a small box with a few atoms,
and try out the commands and look at the resulting viz or dump file.
It should become clear what fix deform is doing.