[lammps-users] CdTe crystal

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1

Dear all

I want to model a crystal of CdTe using Stillinger-weber potential. I
use NVT for 300 kelvin. But after equilibrium some of the Cadmium and
Tellurium atom change their position with each other in the crystal
structure.
Anybody has suggestion?

~pouyan
This is my script:

# CdTe crystal

units metal
boundary p p p

atom_style atomic

lattice custom 3.61 a1 1.0 0.0 0.0 a2 0.0 1.0 0.0 a3 0.0 0.0 1.0 basis
0.0 0.0 0.0 basis 0.5 0.5 0.0 basis 0.5 0.0 0.5 basis 0.0 0.5 0.5
basis 0.25 0.25 0.25 basis 0.75 0.75 0.25 basis 0.75 0.25 0.75 basis
0.25 0.75 0.75
region box block 0 3.0 0 3.0 0 3.0

create_box 2 box

group Cad type 1
group Tel type 2

create_atoms 1 box basis 1 1 basis 2 1 basis 3 1 basis 4 1 basis
5 2 basis 6 2 basis 7 2 basis 8 2

## Instantiate the system.

mass 1 112.40
mass 2 127.60

# Initial velocities correspond to around 1500 kelvin.

velocity all create 1500 98745
pair_style sw
pair_coeff * * CdTe.sw Cd Te

neighbor 2.0 bin
neigh_modify delay 0 every 1 check yes
timestep 0.003

# Equilibrate the system.
fix 1 all nvt temp 1500 300 0.003
thermo 5

dump 1 all atom 3 dump.CdTe

run 2700

2

Assuming you have verified you have created the correct CdTe
lattice structure, I would try using the SW potential first
to verify some property in the literature that this potential
has been used for. If you created a perfect bulk lattice and
run at 300K, it seems hard to believe 2 atoms would swap
positions in a solid. That's too low a T for diffusion to
take place without defects.

Steve

3

Dear Steve

You are write I expect no diffusion in the crystal. CdTe crystal is
like a FCC crystal for cadmium which each cadmium has Te beside it in
(0.25 0.25 0.25) offset. My custom crystal has this structure at first
iteration and I expect the crystal maintain its shape in following
time-steps in 300 kelvin. But in next time-steps the atoms start to
move and the interesting part is that they make new crystal structure
which is not correct. I think the problem is that potential function
does not defer between the Cd and Te atoms. Probably I made a mistake
in defining these 2 types of atoms but I don't find the error in the
script. Would you take a look.

# CdTe Stillinger-weber

# Energy in eV, time in ps, distance in angstroms.
units metal
boundary p p p

atom_style atomic

lattice custom 3.61 a1 1.0 0.0 0.0 a2 0.0 1.0 0.0 a3 0.0 0.0 1.0 basis
0.0 0.0 0.0 basis 0.5 0.5 0.0 basis 0.5 0.0 0.5 basis 0.0 0.5 0.5

basis 0.25 0.25 0.25 basis 0.75 0.75 0.25 basis 0.75 0.25 0.75 basis
0.25 0.75 0.75
region box block 0 2.5 0 2.5 0 2.5
create_box 2 box
create_atoms 2 box basis 1 1 basis 2 1 basis 3 1 basis 4 1 basis
5 2 basis 6 2 basis 7 2 basis 8 2

mass 1 112.40
mass 2 127.60

# Initial velocities correspond to around 300K.

velocity all create 300 98745
pair_style sw
pair_coeff * * CdTe.sw Cd Te

neighbor 2.0 bin
neigh_modify delay 0 every 1 check yes
timestep 0.002

fix 1 all nvt temp 300 300 0.002

thermo 5

dump 1 all atom 3 dump.CdTe

run 1000

4

Sorry, I don't do that level of debugging for people. I would build
a tiny system and look at the dump file or viz it to insure
you got what you expect. Maybe someone else on
the list has used the CdTe potential. You could find the
paper that created it and see if you can reproduce the values
listed in that paper.

Steve