# Question about remap in fix deform

Dear LAMMPS users,

I am using lammps to simulate the p-v curve in phase diagram for a pure substance. Fix nve and langevin are used to control a constant temperature, and fix deform is used to control and vary the volume. I am using a rectangular box, and the deformation is either expanding or compressing the box, while keeping the shape.

As I read from the manual, remap x is used as default in this command.
What does remap mean?
and what is the difference between remap x and the reentry of atoms from the periodic boundary condition?
Also, is there other commands to control and vary the volume other than fix deform?

Thanks!

Dear LAMMPS users,

I am using lammps to simulate the p-v curve in phase diagram for a pure
substance. Fix nve and langevin are used to control a constant temperature,
and fix deform is used to control and vary the volume. I am using a
rectangular box, and the deformation is either expanding or compressing the
box, while keeping the shape.

As I read from the manual, *remap x* is used as default in this command.
What does remap mean?

​from the fix deform documentation:

Each time the box size or shape is changed, the *remap* keyword determines
whether atom positions are remapped to the new box. If*remap* is set
to *x* (the
default), atoms in the fix group are remapped; otherwise they are not. Note
that their velocities are not changed, just their positions are altered. If
*remap* is set to *v*, then any atom in the fix group that crosses a
periodic boundary will have a delta added to its velocity equal to the
difference in velocities between the lo and hi boundaries. Note that this
velocity difference can include tilt components, e.g. a delta in the x
velocity when an atom crosses the y periodic boundary. If *remap* is set to
*none*, then neither of these remappings take place.
Conceptually, setting *remap* to *x* forces the atoms to deform via an
affine transformation that exactly matches the box deformation. This
setting is typically appropriate for solids. Note that though the atoms are
effectively “moving” with the box over time, it is not due to their having
a velocity that tracks the box change, but only due to the remapping. By
contrast, setting *remap* to *v* is typically appropriate for fluids, where
you want the atoms to respond to the change in box size/shape on their own
and acquire a velocity that matches the box change, so that their motion
will naturally track the box without explicit remapping of their
coordinates.​