Why the interaction potential parameters are not working for Iron-oxide nanoparticles?

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1

Dear All,

I am studying several thermodynamics properties of crystalline iron oxides (Fe3O4, Gamma- Fe2O3) in their solid phases using LAMMPS. At the moment, I am facing many difficulties in simulating the nano-particles.
The crystal structures, I am using, are coming from first principle simulations and interaction potentials parametrisations (Buckingham potential for short range and Coulombic potentials for Long range) developed for bulk iron oxides. Clearly the modelling is properly working for bulk iron oxides. Instead, when applied to nano-particles, the nano-particles become amorphous at any temperature.

The inputs for Maghemite system is given below. Bulk is simulated at NPT environment whereas the NP is kept in a very large simulation box with NVT ensemble.

###For Bulk###

#maghemite

1. INITIALIZATION

units metal
boundary p p p
log log.maghemite
atom_style full
pair_style hybrid/overlay buck 10.0 coul/long 20.0
bond_style none
angle_style none
dihedral_style none
improper_style none
kspace_style pppm 0.0001
pair_modify shift yes

2. ATOM AND BOX DEFINITION

read_data data.maghemite_supercell
pair_coeff * * coul/long
pair_coeff 1 2 buck 1414.6 0.3128 0
pair_coeff 2 2 buck 9547.96 0.2192 32

3. MD SETTINGS

neighbor 2.0 bin
neigh_modify delay 5 check yes
replicate 9 9 3
timestep 0.001
restart 500 maghemite_restart_equil_1 maghemite_restart_equil_2
thermo_style custom step temp etotal press lx ly lz density enthalpy vol
thermo 1000
dump 1 all atom 5000 maghemite_nano_solid_vis_equil.dump

4. MD RUN

#----------Equilibration---------
fix 2 all npt temp 280.00 280.00 0.1 aniso 1.0 1.0 1.0
run 500000
unfix 2

###For NP###

#maghemite NP

1. INITIALIZATION

units metal
boundary p p p
log log.ti
atom_style full
pair_style hybrid/overlay buck 10.0 coul/long 10.0
bond_style none
angle_style none
dihedral_style none
improper_style none
kspace_style pppm 0.0001
pair_modify shift yes

2. ATOM AND BOX DEFINITION

read_data data.maghemite_nano_cube
pair_coeff * * coul/long
pair_coeff 1 2 buck 1414.6 0.3128 0
pair_coeff 2 2 buck 9547.96 0.2192 32

3. MD SETTINGS

neighbor 2.0 bin
neigh_modify delay 5 check yes
timestep 0.001
restart 500 maghemite_restart_equil_1 maghemite_restart_equil_2
thermo_style custom step temp etotal press lx ly lz density enthalpy vol
thermo_modify flush yes
thermo 1000
dump 1 all atom 5000 maghemite_nano_solid_vis_equil.dump

4. MD RUN

#----------Equilibration---------
fix 2 all nvt temp 280.00 280.00 0.1
run 500000
unfix 2

In my opinion, the lack of Coulombic contribution in the nano-system (for bulk the contribution is large as the simulation box is periodic in each dimension) is causing this behavior.

Has anyone ever faced such issue while studying crystalline nano-structures? Any tips or suggestions would be much appreciated.

Sincerely,
Sankhadeep Bose

data.maghemite_supercell (17 KB)

data.maghemite_nano_cube (257 KB)

2

Dear All,

I am studying several thermodynamics properties of crystalline iron oxides (Fe3O4, Gamma- Fe2O3) in their solid phases using LAMMPS. At the moment, I am facing many difficulties in simulating the nano-particles.
The crystal structures, I am using, are coming from first principle simulations and interaction potentials parametrisations (Buckingham potential for short range and Coulombic potentials for Long range) developed for bulk iron oxides. Clearly the modelling is properly working for bulk iron oxides. Instead, when applied to nano-particles, the nano-particles become amorphous at any temperature.

no big surprise here. you are using a model and potential parameterization that is not suitable for non-bulk systems.
for a bulk system and mean field approach is often sufficient and that is what you are using, but once you introduce surfaces into your system, you will need something more complex and flexible. you need to study the literature for models/potentials that are specifically suitable of surfaces and nanoparticles.

axel.

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