[lammps-users] fix deform command

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1

Dear Lammps Users,

I have a question about the ‘fix deform’ command of Lammps. According to the manual, the ‘erate’ argument
changes the specified dimension (for example x length) of the box via “constant displacement”.
Also, if the ‘volume’ argument is chosen for other two dimensions, then both the y,z box lengths will shrink as x increases
to keep the volume constant (product of x,y,z lengths). This means the all three boundaries are moving linearly
or with a constant velocity at each time step of a specific number of run.

If so, then my question is how are the velocity components of each atom inside the box determined at each time step? Do the
atoms also move with a constant boundary velocity in the respective direction during the whole run
or the constant boundary velocities are assigned to each atom at the beginning
of the simulation and then the atomic positions and velocity components are updated at each time step following the Velocity-Verlet algorithm?

I would really appreciate any suggestion.

Thank you.
Ishraq Shabib

2

See the "remap" option for fix deform. The doc page explains
how it affects the position and velocity of atoms as the box deforms.

Steve

3

Hi Steve,

thanks for your comments. I understand that the atom positions are remapped to the new box each time the box size or shape is changed. So, during the dynamics (or in the Velocity-Verlet integration), only the velocity and acceleration of the atoms are updated in each time step, not the atomic positions. is that right?

thanks,
Ishraq.

4

No on both counts. The atom positions are only remapped if you use
remap x with box deform. And the NVE integration will further change
the atom positions.

Steve