[lammps-users] question about creating atioms

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1

Dear everyone:
I have a question about creating atoms, and my input file is as follows:

boundary         s s s 
units            metal
atom_style       atomic
lattice          fcc    3.743

region           box sphere 0 0 0 5.555555556

create_box       2 box
create_atoms     1

mass             1 58.69
mass             2 195.08

region           core sphere 0 0 0 5.65
group            core region core
group            shell subtract all core

set              core atom 1
set              shell atom 2

The output file is:
Created 6083 atoms
3043 atoms in group core
3040 atoms in group shell
Setting atom values ...
  3043 settings made
Setting atom values ...
  3040 settings made
My question is why should I choose a larger radius of core (comparing with the box's )to get the proper ratio of atom numbers.
In fact, when I choose a smaller radius of core, the number of core atoms is
 small.
How does the program work when I type these commands?
       Best regards,
2

The simulation box is always a box, not a sphere. So when
you create_atoms in a box that encloses the sphere. I.e.
a box of side length 2*5.55.

If you want to just create a sphere of atoms you would
need

create_atoms 1 region box

I suggest you visualize the atoms you are creating and
it will be obvious.

Steve