Should periodic boundary create constant volume?

Hello,

I am using a "boundary p p p " command with a simulation, and I have a non-constant volume. However, for a periodic boundary, shouldn’t the volume remain constant, since when a particle leaves the simulation box it reappears on the other side? I would assume that a periodic boundary simulation would have a constant volume no matter what the other parameters are.
Can you please explain to me my error?

Thank you very much for your time.
Casey

Casey Hansen
[email protected]…24…
(724) 674 2634

Hello,

I am using a "boundary p p p " command with a simulation, and I have a
non-constant volume. However, for a periodic boundary, shouldn't the volume
remain constant, since when a particle leaves the simulation box it
reappears on the other side? I would assume that a periodic boundary
simulation would have a constant volume no matter what the other parameters
are.

true, unless you use a fix like fix npt or fix box/relax that can
change the box volume.

Can you please explain to me my error?

impossible to say with certainty without seeing the rest of your input.

axel.

Thank you for your response. I am using a fix nph iso. Is this fix forcing the volume to change because of how pressure is calculated? And does this mean that the particles are still moving across the periodic boundary as usual? My input is below.

units metal
boundary p p p
atom_style atomic
lattice diamond 5.430
region rod cylinder x 0 0 5 -2.5 2.5
region rod2 cylinder x 0 0 5 -2.5 2.5 side out
create_box 1 rod
create_atoms 1 region rod
mass 1 28.085
pair_style tersoff
pair_coeff * * Si.tersoff Si
variable vx atom 1.00vx
variable vy atom 1.00
vy
variable vz atom 1.00*vz
velocity all create 300.0 87287
velocity all set v_vx v_vy v_vz
fix 1 all nph iso 10.0 10.0 1000.0
fix_modify 1 energy yes
fix 2 all press/berendsen iso 10.0 10.0 1000.0
thermo_style custom step temp vol etotal press
thermo 10
timestep 0.001
run 10000

Thanks,
Casey

Thank you for your response. I am using a fix nph iso. Is this fix forcing
the volume to change because of how pressure is calculated? And does this

fix nph does change the volume. this is documented. it has nothing to
do with "how the pressure is calculated", the current instantaneous
pressure is an input to fix nph.

mean that the particles are still moving across the periodic boundary as
usual? My input is below.

dear casey,

please understand that this mailing list not a "MD beginner training
service". the questions you are asking are fundamental MD questions
and you should first talk to your adviser/supervisor and your more
experienced colleagues. your questions indicate that you are lacking
some rather fundamental understanding of what you are doing, and this
is extremely inefficient and time consuming to resolve over e-mail.
more importantly, people that respond to e-mails here prefer to spend
their effort on resolving LAMMPS-specific problems, discussing
advanced usage options and new/missing/broken features.

thanks for your understanding,
     axel.