Hi,
I am having an issue in the reproduction of a vacancy migration energy barrier in pure Ni (FCC). I am using a FeNiCr eam potential (https://doi.org/10.1088/0965-0393/21/8/085004) in which it is said that the migration energy barrier of a vacancy in pure nickel is 1.09 eV. Also, another paper confirmed that using the LAMMPS neb method with the same potential the energy barrier is about 1.09 eV (https://doi.org/10.1016/j.actamat.2017.12.064). Also, other literature says the vacancy migration barrier is about 1.04-1.09 eV. However, after several attempts with different settings I get an energy of 0.93 which does not change very much by changing settings. What I do is:
Set a box of size 10x10x10 lattice constants
Relax it for thousands of timesteps with the fix npt @ 300 K and 0 bar
minimization
Get the actual lattice constant
Get the data file that I will use as initial.Nivac file for the neb method
Set another input file as follows:
Read the initial.Nivac data file
Remove one atom in the bulk of the box
Generate a final.Nivac file in which I move 1 nearest neighbor of the removed atom to the position of the removed atom
Set the box temperature to 300 K (tried also different Temperatures)
Relax the box with the fix npt
minimization
Run the neb method as follows:
Fix neb all 1.0 (I tried with neb “smallergroupofatoms” and also different settings, like perp, parallel, different kspring values etc)
Timestep 0.01
Minimization style quickmin (tried also fire)
Neb 0 0.01 1000 500 10 final final.Nivac (tried with several different values of etol ftol N1 N2 Nevery parameters)
(I usually run partitions like this: 10x1, 8x2, 8x4)
I then plot all the “PEN” values of the final line of the log.lammps file making the difference (PEN – min(PEx))
I always get the maximum value of such difference between 0.9 and 0.94 eV. Still, literature says I should get higher values, especially with the potential I’m using.
(I also dumped the files and I actually see the chosen NN of the vacancy moving to the vacancy spot leaving the new vacancy where it was.)
I would like to know if there is something wrong in what I’m doing or if there are some settings I am missing.
I can attach the in. initial. And final. Files if it can be useful.
Thank you.
Cheers,
Stefano Segantin