Hi all,
I have been having some issues with the MEAM potential in LAMMPS. I have been attempting to use MEAM to model Ni-Si interactions, and have been using the implementation discussed by Baskes et al in the paper “Atomistic calculations of composite interfaces” in the journal Modelling Simul. Mater. Sci. Eng. 2 (1994) 505-518.
According to Greg Wagner in a previous post I made (http://lammps.sandia.gov/threads/msg36200.html), the “l12” structure is properly coded in LAMMPS and the Ni-Si interactions can be easily modeled by using the parameters given in the paper. Previously, I have been using their parameters to model amorphous-Si/Ni interactions, the structure of which deviates significantly from the reference structures used to define the interactions. Although the behavior of the Ni-Si interactions has been questionable, I thought that that would be primarily due to the use of the Baskes Ni-Si parameters for a system far from the reference structure. However, it appears that if I use the Baskes parameters, I cannot even correctly describe the reference structure from which they were defined (Ni3Si). Can anyone tell me if I am doing something wrong or why the Baskes Ni-Si parameters are unable to model the reference Ni-Si structure?
I’ll include a simple input script that is just a block of Ni3Si sitting at 300K. The parameters I’m using match those in the Baskes paper, but the system quickly loses its crystal structure (regardless of the equilibration) and goes unstable (rapid rise in temperature and energy) depending on the length of equilibration. But a pure Ni (fcc) and pure Si (diamond) block maintain their structure and stability without any equilibration. Any ideas would be greatly appreciated.
Michael Price
P.S. I have been using the 9-Nov-13 version of LAMMPS (behavior is the same using earlier versions as well)
LAMMPS input script:
#-------------SETUP------------------------------------------
variable outputfolder string output
shell mkdir ${outputfolder}
dimension 3
boundary p p p
units metal
atom_style atomic
neighbor 2.0 bin
neigh_modify delay 5
lattice fcc 3.405
region box block 0 10 0 10 0 10
create_box 2 box
mass 1 58.71 # Ni
mass 2 28.086 # Si
pair_style meam
pair_coeff * * library.meam Ni1 Si2 NiSi.meam Ni1 Si2
#-------------CREATE ATOMS-----------------------------------
create_atoms 1 region box basis 1 2 basis 2 1 basis 3 1 basis 4 1
#-------------INIT-------------------------------------------
velocity all create 300 1272
fix int all nve
timestep 0.0001
thermo 50
thermo_style custom step temp epair emol etotal press
dump 1 all custom 100 ${outputfolder}/dump.stats id type x y z
dump_modify 1 sort id
dump 2 all cfg 100 ${outputfolder}/conv*.cfg mass type xs ys zs
dump_modify 2 sort id
dump_modify 2 element Ni Si
#-------------RUNS-------------------------------------------
System quickly goes unstable unless there is a fairly long (>10ps)
equilibration process and small (.00025 ps) timestep. A pure
Ni (fcc) or pure Si (dia) lattice is stable even without
equilibration
fix equilibrate all temp/rescale 1 300 300 1 1
#fix equilibrate all langevin 300 300 .01 39857
run 12000
unfix equilibrate
run 1000
--------------------------------------------MEAM FILES----------------------------
library.meam
meam data from vax files fcc,bcc,dia 11/4/92
elt lat z ielement atwt
alpha b0 b1 b2 b3 alat esub asub
t0 t1 t2 t3 rozero ibar
‘Ni1’ ‘fcc’ 12. 28 58.71
4.99 2.45 2.20 6 2.20 3.52 4.45 1.10
1.0 3.57 1.60 3.70 1.0 0
‘Si2’ ‘dia’ 4. 14 28.086
4.87 4.4 5.5 5.5 5.5 5.431 4.63 1.
1.0 3.13 4.47 -1.80 1 0
NiSi.meam
rho0(1) = 4.88
rho0(2) = 1
delta(1,2) = 0.36
alpha(1,2) = 6.00
re(1,2) = 2.41
lattce(1,2) = l12