# [lammps-users] shape and volume

Dear all,

The problem I am working on consists of a box of water molecules between two solid walls of silicon (parallel to the yz plane) in the x direction. Periodic boundaries are applied along y and z directions, while fixed boundaries exist in the x-direction. Now, the two walls are squeezed in the x-direction by a constant delta-x. It is required that the volume of the water box remain constant, and so the change in delta-x be compensated by subsequent/proportional increase along y and z directions. I was looking at the fix box/relax command, but could not be sure if that would be the right option. Any suggestions about a suitable command for the above will be very helpful.

Thank you.

Ganesh

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You don’t say how you plan to compress the x dimension, but you can
use fix deform to adjust the y and z (as well as x) dimensions in
a variety of ways.

Steve

Dear all,

The problem I am working on consists of a box of water molecules
between two solid walls of silicon (parallel to the yz plane) in the x
direction. Periodic boundaries are applied along y and z directions,
while fixed boundaries exist in the x-direction. Now, the two walls
are squeezed in the x-direction by a constant delta-x. It is required
that the volume of the water box remain constant, and so the change in
delta-x be compensated by subsequent/proportional increase along y and
z directions. I was looking at the fix box/relax command, but could
not be sure if that would be the right option. Any suggestions about a
suitable command for the above will be very helpful.

Thank you.

Ganesh

dear ganesh,

i think what you want to do is not possible to do the way you
describe it. if you want to keep the water volume constant _and_
use periodic boundaries in y and z, then you have to stretch
the si-walls, which sounds _very_ unreasonable. i am also wondering
what kind of macroscopic system that would model?

if, however, you want to model a microscopic system, then it would
be _much_ more reasonable to just use large walls and no periodic
boundaries at all.

if the system would be so that it is large in y- and z-dimensions
and small in x-, then having y and z fixed with PBC would sound
much more reasonable and realistic to me, as i would expect the
water density increase under those circumstances, at least on MD
timescales.

cheers,
axel.