# fix deform command

Dear LAMMPS users,

I would to calculate the elasticity of polymers by using the fix deform command. The simulation box is unixially deformed along x axis. I have two questions about the fix deform +NVT command.  The first is that when the box is deformed,the corresponding stress responsive to the deformed box can be obtained, why don't we wait the deformed box balance instead of stretching the box  continually. The second is  why the temperture increase  gradually with stretching the box incessantly even though the fix NVT commmand is applied coupled with the fix deform command.
Can someone help me with these questions,it will help a lot.

Alehmn

Dear LAMMPS users,

I would to calculate the elasticity of polymers by using the fix deform command. The simulation box is unixially deformed along x axis. I have two questions about the fix deform +NVT command.
The first is that when the box is deformed,the corresponding stress responsive to the deformed box can be obtained, why don't we wait the deformed box balance instead of stretching the box  continually.

then you would not be using fix deform, but change_box, as demonstrated by the examples for computing elastic constants in the LAMMPS distribution. fix deform is a continuous prescribed deformation. that is what it was written for. so you have to as the question why to use fix deform to wherever you picked up the method that you are using.

The second is  why the temperture increase  gradually with stretching the box incessantly even though the fix NVT commmand is applied coupled with the fix deform command.

if you are deforming your system, you are adding energy to it (you increase the potential energy which then will then in part relax into kinetic energy). depending on how quickly you deform your system, a different fraction of this energy can be transported away from the system via the thermostat. fix nvt is specifically designed for equilibrium systems and thus not every effective in exchanging large amounts of energy quickly. but then again, your deformation speed will also impact the accuracy of your results. so you have to find an optimum here.

axel.