Incorrect crystal orientation on outside of periodic simulation box

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1

Hi,

I am having some issues understanding why atoms around the edge of a periodic simulation box are not in BCC orientation when I orient outside the typical [100] orientations. I have done some reading on this and found that the box should be a multiple of the minimum periodic distance. Periodic distance is then calculated as (SQRT(h^2 + k^2 + l^2). For example the [1 1 0] orientation would be a min periodic distance of (sqrt(2). I tried testing this for the iron BCC structure below and noticed that the default LAMMPS spacing was unexpected for the z direction. As such I modified the lattice command with the spacing term as below. I think make a region that is 20 x 20 0 x 20 with lattice units. Spacing in this case is 4.964, 4.05314. 7.0204 and box size is 99.28, 81.1, 140.4 as expected. However I am still seeing a few atoms on the outer layer not being in the expected BCC orientation (97% BCC).

What am I missing?

------------------------ INITIALIZATION ----------------------------

units metal
dimension 3
boundary p p p
atom_style atomic
atom_modify map hash
variable latparam equal 2.866

----------------------- ATOM DEFINITION ----------------------------

lattice bcc ${latparam} orient x 1 1 1 orient y 1 -1 0 orient z 1 1 -2 spacing 1.732051 1.414214 2.44949
region whole block 0 20 0 20 0 20
create_box 1 whole
create_atoms 1 region whole
mass 1 55.85

Thanks
Liam

2

Hi,

Due to numerical precision/error, the periodicity can be subtile to achieve.
Lammps take care of this, but only if you use “create_atoms" with the style “box”.

So, I expect your parameters to creat a fully periodic crystal if you replace

create_atoms 1 region whole

by

create_atoms 1 box

Julien

3

More details on this are given on the create_atoms doc page, for box
style vs region style.

Steve

4

Thanks to both of you. By using the box command and manually changing the spacing it is now perfect.

Appreciate the help.

Liam