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[lammps-users] Applying strain to fcc gold modeled using lennard jones potential at 50 kelvin
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[lammps-users] Applying strain to fcc gold modeled using lennard jones potential at 50 kelvin

From: melika_ bm85 <melika_bm85@...16...>
Date: Tue, 27 Jun 2017 08:58:36 +0000 (UTC)

Dear Users,
I have modeled gold at 50 kelvin temperature using lennard jones as the interatomic potential. My simulation box is 20*20*20 angstrom. I let the box relax in NVT at 50 kelvin for 100 picoseconds.
Here is my lammps code:
units metal
dimension 3
boundary p p p
atom_style atomic
lattice fcc 4.0782
region box prism 0 20 0 20 0 20 0.0 0.0 0.0 units box
create_box 1 box
create_atoms 1 box
group box region box
mass 1 196.97
pair_style lj/cut 10
pair_coeff * * 0.00169117 2.934
pair_write 1 1 1000 r 0.01 19.990 potential.table.txt lj/cut
neighbor 0.3 bin
neigh_modify delay 5
compute csym all centro/atom fcc 
compute peratom all pe/atom 
reset_timestep 0
thermo 1000
timestep 0.001
thermo_style custom step temp pe lx ly lz press pxx pyy pzz 
min_style cg
minimize 1e-15 1e-15 10000 100000
reset_timestep 0
velocity all create 50 4928459 mom yes rot yes dist gaussian
fix 1 all nvt temp 50 50 1 drag 1
dump 1 all custom 250 dump.tensile.atom id type xs ys zs 
compute myRDF all rdf 1000
fix radi all ave/time 100 100 10000 c_myRDF[*] file tmp.rdf mode vector
run 100000
unfix 1
variable tmp equal "lx"
variable L0 equal ${tmp}
print "initial length, L0: ${L0}"
fix 2 all nve
variable srate equal 1.0e10
variable srate1 equal "v_srate / 1.0e12"
fix 3 all deform 1 x erate ${srate1} units box remap x
variable strain equal "(lx - v_L0)/v_L0"
variable p1 equal "v_strain"
variable p2 equal "-pxx/10000"
variable p3 equal "-pyy/10000"
variable p4 equal "-pzz/10000"
fix def1 all print 100 "${p1} ${p2} ${p3} ${p4}" file Au_tn_100.def1.txt screen no
dump 2 all custom 250 dump.tensile.atom.axial id type xs ys zs c_csym c_peratom fx fy fz
thermo 1000
thermo_style custom step v_strain temp v_p2 v_p3 v_p4 ke pe press
run 40000

The problem I have is that the stress-strain curve that I get at the end of this simulation is not what I am expecting and when I carry out the exact same code with eam potential instead I get a completely different curve which looks very fine and correct and how it should be. I have attached both curves to this email. I want to ask if the problem is with how I am applying the strain to my box and whether I need to change my code to get the correct result or whether lj potential is unable to give a correct result for fcc crystals and I should not bother with it any more. The file attached is only 52KB. I hope it is not inconvenient. 
Melika Vokhshoori

Attachment: curve.xlsx
Description: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet