[lammps-users] minimum image criteria

If your cutoff is 3.77, then it is not less
than 1/2 the size of the box for the
case box size = 4, that was in your initial
message. Thus when you change the
box size from 4 to 6, you are probably
losing a periodic interaction between
the 2 atoms.

Steve

Steve,

Thanks for your reply.
The minimum image criteria says that if the distance between two atoms
in a box is less than the 1/2 the size of the box, the periodic
interaction should not be considered. But I think lammps doesn't care
for that.
Your answer also says that in the case box size=4 there is a periodic
interaction between atoms. but the minimum distance between atoms is
0.5.
I also don't see any code in lammps that does the min image computation.

Best regards,
Mohyeddin

Steve,

Thanks for your reply.
The minimum image criteria says that if the distance between two atoms
in a box is less than the 1/2 the size of the box, the periodic
interaction should not be considered. But I think lammps doesn't care
for that.

lammps _does_ care, but it _also_ cares about _additional_ images
of particles due to the way it does the domain decomposition and
building of the neighbor lists. since if your box is so small that it would
contain multiple copies of your atoms, they will be considered for
computing the forces and energies as "ghost" atoms.

Your answer also says that in the case box size=4 there is a periodic
interaction between atoms. but the minimum distance between atoms is
0.5.

but there are additional interactions with images of those particles within
the cutoff range.

I also don't see any code in lammps that does the min image computation.

this is implicit when the neighbor lists are built. the system is divided into a
grid of bins and atoms are sorted into those bins and then the neighbor
lists are computed from those binned positions.

axel.

Specifically, if your box size is 4 and 2 atoms are 0.5 apart, then
one of the atoms is 3.5 away from an image of the other. Since
3.5 < 3.77 which is your cutoff, then there are image interactions
that LAMMPS will compute.

Steve