# occupying the interstitial atoms

Dear Axel and lammps-users

i am trying to occupy carbon atoms in my simulation box in the interstitial positions. but i am not sure what should i use in create_atoms command? is the random word the right one?

Thanks for your help.

random will give you an amorphous system, which is not what you want. If you have just a few interstitial atoms, you can enter them by hand by editing a data file or using create_atoms single. If you want to place a large number of interstitial atoms, I suggest the following.

1. Identify the lattice structure that includes both the original lattice and the interstitial sites. For example, I believe that BCC Fe and octahedral interstitial sites together make a simple cubic lattice, with the Fe atoms lying on the even sites and insterstitials on the odd sites.
2. Build a system with all sites occupied, in the manner that is described for Wurtzite in lattice.html. These instructions were mysteriously truncated, so I repeat them below.
3. Delete the desired fraction of interstitial atoms using the delete_atoms porosity command

Aidan

Here are the complete commands for Wurtzite, now posted at lattice.html:

``````variable a equal  4.340330
variable b equal  \$a*sqrt(3.0)
variable c equal  \$a*sqrt(8.0/3.0)

``````
``````variable 1_3 equal 1.0/3.0
variable 2_3 equal 2.0/3.0
variable 1_6 equal 1.0/6.0
variable 5_6 equal 5.0/6.0
variable 1_12 equal 1.0/12.0
variable 5_12 equal 5.0/12.0

``````
``````lattice custom    1.0     &
a1      \$a      0.0     0.0     &
a2      0.0     \$b      0.0     &
a3      0.0     0.0     \$c      &
basis   0.0     0.0     0.0     &
basis   0.5     0.5     0.0     &
basis   \${1_3}  0.0     0.5     &
basis   \${5_6}  0.5     0.5     &
basis   0.0     0.0     0.625   &
basis   0.5     0.5     0.625   &
basis   \${1_3}  0.0     0.125   &
basis   \${5_6}  0.5     0.125

``````
``````region myreg block 0 1 0 1 0 1
create_box      2 myreg
create_atoms    1 box      &
basis   5       2       &
basis   6       2       &
basis   7       2       &
basis   8       2
``````
``````
``````