The first step is to relax the structure with ReaxFF, which which will be achieved using molecular dynamics. To ensure the system equilibrates properly, we will monitor certain parameters over time, such as the system volume.
Create a folder if needed and place the initial input file, relax.lmp, into it. Then, open the file in a text editor of your choice, and copy the following into it:
units real
atom_style full
read_data silica.data
If you are using LAMMPS-GUI
To begin this tutorial, select Start Tutorial 5 from the
Tutorials menu of LAMMPS--GUI and follow the instructions.
The editor should display the following content corresponding to relax.lmp
So far, the input is very similar to what was seen in the previous tutorials.
Some basic parameters are defined (units and atom_style),
and a .data file is imported by the read_data command.
The initial topology given by silica.data
is a small amorphous silica structure. This structure was created using a force field called
Vashishta :cite:`vashishta1990interaction`. If you open the silica.data
file, you will find in the Atoms section that all silicon atoms have a
charge of q = 1.1\,\text{e}, and all oxygen atoms have a charge of q = -0.55\,\text{e}.
Note
Assigning the same charge to all atoms of the same type is common with many force fields, including the force fields used in the previous tutorials. This changes once ReaxFF is used: the charge of each atom will adjust to its local environment.
Next, copy the following three crucial lines into the relax.lmp file:
pair_style reaxff NULL safezone 3.0 mincap 150
pair_coeff * * reaxCHOFe.inc Si O
fix myqeq all qeq/reaxff 1 0.0 10.0 1.0e-6 reaxff maxiter 400
In this case, the pair_style reaxff is used without a control file. The
safezone and mincap keywords are added to prevent
allocation issues, which sometimes can trigger segmentation faults and
bondchk errors. The pair_coeff command uses the reaxCHOFe.inc
file, which should have been downloaded during the tutorial set up. Finally, the
fix qeq/reaxff is used to perform charge equilibration :cite:`rappe1991charge`,
which occurs at every step. The values 0.0 and 10.0 represent the
low and the high cutoffs, respectively, and 1.0 \text{e} -6 is the tolerance.
The maxiter sets an upper limit to the number of attempts to
equilibrate the charge.
Next, add the following commands to the relax.lmp file to track the evolution of the charges during the simulation:
group grpSi type Si
group grpO type O
variable qSi equal charge(grpSi)/count(grpSi)
variable qO equal charge(grpO)/count(grpO)
variable vq atom q
To print the averaged charges qSi and qO using the
thermo_style command, and create images of the system. Add the
following lines to relax.lmp:
thermo 100
thermo_style custom step temp etotal press vol v_qSi v_qO
dump viz all image 100 myimage-*.ppm q type shiny 0.1 box no 0.01 view 180 90 zoom 2.3 size 1200 500
dump_modify viz adiam Si 2.6 adiam O 2.3 backcolor white amap -1 2 ca 0.0 3 min royalblue 0 green max orangered
Here, the atoms are colored by their charges q, ranging from royal blue
(when q=-1\,\text{e}) to orange-red (when q=2\,\text{e}).
We can generate histograms of the charges for each atom type using
fix ave/histo commands:
fix myhis1 grpSi ave/histo 10 500 5000 -1.5 2.5 1000 v_vq file relax-Si.histo mode vector
fix myhis2 grpO ave/histo 10 500 5000 -1.5 2.5 1000 v_vq file relax-O.histo mode vector
We can also use the fix reaxff/species to evaluate what species are
present within the simulation. It will be useful later when the system is deformed,
and bonds are broken:
fix myspec all reaxff/species 5 1 5 relax.species element Si O
Here, the information will be printed every 5 steps in a file called relax.species. Let us perform a very short run using the anisotropic NPT command and relax the density of the system:
velocity all create 300.0 32028
fix mynpt all npt temp 300.0 300.0 100 aniso 1.0 1.0 1000
timestep 0.5
run 5000
write_data relax.data
Run the relax.lmp file using LAMMPS. As seen from relax.species,
only one species is detected, called O384Si192, representing the entire system.
As the simulation progresses, the charge of every atom fluctuates because it is adjusting to the local environment of the atom. It is also observed that the averaged charges for silicon and oxygen atoms fluctuate significantly at the beginning of the simulation, corresponding to a rapid change in the system volume, which causes interatomic distances to shift quickly. The atoms with the most extreme charges are located at structural defects, such as dangling oxygen groups.
Finally, the generated .histo files can be used to plot the probability distributions, P(q).
Let us apply a deformation to the structure to force some \text{Si}-\text{O} bonds to break (and eventually re-assemble). Open the deform.lmp file, which must contain the following lines:
units real
atom_style full
read_data relax.data
pair_style reaxff NULL safezone 3.0 mincap 150
pair_coeff * * reaxCHOFe.inc Si O
fix myqeq all qeq/reaxff 1 0.0 10.0 1.0e-6 reaxff maxiter 400
group grpSi type Si
group grpO type O
variable qSi equal charge(grpSi)/count(grpSi)
variable qO equal charge(grpO)/count(grpO)
variable vq atom q
thermo 200
thermo_style custom step temp etotal press vol v_qSi v_qO
dump viz all image 100 myimage-*.ppm q type shiny 0.1 box no 0.01 view 180 90 zoom 2.3 size 1200 500
dump_modify viz adiam Si 2.6 adiam O 2.3 backcolor white amap -1 2 ca 0.0 3 min royalblue 0 green max orangered
fix myhis1 grpSi ave/histo 10 500 5000 -1.5 2.5 1000 v_vq file deform-Si.histo mode vector
fix myhis2 grpO ave/histo 10 500 5000 -1.5 2.5 1000 v_vq file deform-O.histo mode vector
fix myspec all reaxff/species 5 1 5 deform.species element Si O
The only difference with the previous relax.lmp file is the path to the relax.data file.
Next, let us use fix nvt instead of fix npt to apply a
Nosé-Hoover thermostat without a barostat:
fix mynvt all nvt temp 300.0 300.0 100
timestep 0.5
Here, no barostat is used because the change in the box volume will be imposed
by the fix deform.
Let us run for 5000 steps without deformation, then apply the fix deform
to progressively elongate the box along the x-axis during 25000 steps. Add
the following line to deform.lmp:
run 5000
fix mydef all deform 1 x erate 5e-5
run 25000
write_data deform.data
Run the deform.lmp file using LAMMPS. During the deformation, the charge values progressively evolve until the structure eventually breaks down. After the structure breaks down, the charges equilibrate near new average values that differ from the initial averages. The difference between the initial and the final charges can be explained by the presence of defects, as well as new solid/vacuum interfaces, and the fact that surface atoms typically have different charges compared to bulk atoms. You can also see a sharp increase in temperature during the rupture of the material.
You can examine the charge distribution after deformation, as well as during deformation. As expected, the final charge distribution slightly differs from the previously calculated one. If no new species were formed during the simulation, the deform.species file should look like this:
# Timestep No_Moles No_Specs O384Si192
5 1 1 1
(...)
# Timestep No_Moles No_Specs O384Si192
30000 1 1 1
Sometimes, \text{O}_2 molecules are formed during the deformation. If this occurs,
a new column O2 appears in the deform.species file.
Under ambient conditions, some of the surface \text{SiO}_2 atoms become chemically passivated by forming covalent bonds with hydrogen (H) atoms :cite:`sulpizi2012silica`. We will add hydrogen atoms randomly to the cracked silica and observe how the system evolves. To do so, we first need to modify the previously generated data file deform.data and make space for a third atom type. Copy deform.data, name the copy deform-mod.data, and modify the first lines of deform-mod.data as follows:
576 atoms
3 atom types
(...)
Atom Type Labels
1 Si
2 O
3 H
Masses
Si 28.0855
O 15.999
H 1.008
(...)
Open the decorate.lmp file, which must contain the following lines:
units real
atom_style full
read_data deform-mod.data
displace_atoms all move -12 0 0 # optional
pair_style reaxff NULL safezone 3.0 mincap 150
pair_coeff * * reaxCHOFe.inc Si O H
fix myqeq all qeq/reaxff 1 0.0 10.0 1.0e-6 reaxff maxiter 400
The displace_atoms command is used to move the center of the
crack near the center of the box. This step is optional but makes for a nicer
visualization. A different value for the shift may be needed in
your case, depending on the location of the crack. A difference with the previous
input is that three atom types are specified in the pair_coeff command, i.e.
Si O H.
Then, let us adapt some familiar commands to measure the charges of all three types of atoms, and output the charge values into log files:
group grpSi type Si
group grpO type O
group grpH type H
variable qSi equal charge(grpSi)/count(grpSi)
variable qO equal charge(grpO)/count(grpO)
variable qH equal charge(grpH)/(count(grpH)+1e-10)
thermo 5
thermo_style custom step temp etotal press v_qSi v_qO v_qH
dump viz all image 100 myimage-*.ppm q type shiny 0.1 box no 0.01 view 180 90 zoom 2.3 size 1200 500
dump_modify viz adiam Si 2.6 adiam O 2.3 adiam H 1.0 backcolor white amap -1 2 ca 0.0 3 min royalblue 0 green max orangered
fix myspec all reaxff/species 5 1 5 decorate.species element Si O H
Here, the +1 \mathrm{e}{-10} was added to the denominator of the variable qH
to avoid dividing by 0 at the beginning of the simulation. Finally, let us
create a loop with 10 steps, and create two hydrogen atoms at random locations at
every step:
fix mynvt all nvt temp 300.0 300.0 100
timestep 0.5
label loop
variable a loop 10
variable seed equal 35672+${a}
create_atoms 3 random 2 ${seed} NULL overlap 2.6 maxtry 50
run 2000
next a
jump SELF loop
Run the simulation with LAMMPS. When the simulation is over, it can be seen from the decorate.species file that all the created hydrogen atoms reacted with the \text{SiO}_{2} structure to form surface groups (such as hydroxyl (-OH) groups).
(...)
# Timestep No_Moles No_Specs H20O384Si192
20000 1 1 1
At the end of the simulation, hydroxyl (-OH) groups can be seen at the interfaces.
