I am encountering a strange issue when running the same simulation for two systems of different sizes. As shown in the attached image, the monolayer starts bending significantly in the z-direction, forming large out-of-plane waves. The sheet should remain relatively flat in the xy plane, but instead it develops strong curvature and eventually collapses even before applying the deformation.
I am using exactly the same input script, potential, and simulation settings. When I run a 120 × 120 monolayer, the simulation runs without any file errors and the results look reasonable. However, when I run a 150 × 150 monolayer using the same setup, the atoms collapse after the two ensembles (see attached image).
I would like to understand what might be causing this behavior.
To clarify a few points:
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Both systems were generated from the same primitive unit cell using Atomsk. The smaller and larger monolayers were created with the same procedure using atomsk, so I believe the structure generation itself is not the issue.
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Initially, I did not include energy minimization or neighbor settings and didn’t assign any initial velocities in the input script. The paper from which I adopted the initialization conditions also did not include these steps, and the 120 × 120 system worked fine without them. So I tried running the larger system with and without these modification but the same issue still occurs.
3. I also tested different boundary conditions such as shrink-wrapped and fixed boundaries, but the issue still persists.
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A sufficient vacuum region was provided in the z direction since this is a monolayer system, although I am not sure whether this is relevant to the problem.
Here is the input script
# Uniaxial tensile test for Ti2N
##---------------INITIALIZATION-------------------------------
units metal
dimension 3
boundary p p p
atom_style atomic
newton on
##---------------ATOM DEFINITION------------------------------
read_data Ti2N_main_data_file.lmp
##---------------FORCE FIELDS---------------------------------
pair_style meam
pair_coeff * * library.meam Ti N TiN.meam Ti N
##---------------NEIGHBOR SETTINGS----------------------------
neighbor 2.0 bin
neigh_modify every 1 delay 0 check yes
##---------------ENERGY MINIMIZATION--------------------------
min_style cg
minimize 1e-10 1e-10 1000 10000
##---------------INITIAL VELOCITIES ---------------------
velocity all create 300 12345 dist gaussian rot yes
##---------------SETTINGS-------------------------------------
timestep 0.001
variable ts equal 0.001
##---------------COMPUTES-------------------------------------
compute 1 all stress/atom NULL
compute 2 all reduce sum c_1[1] c_1[2]
variable Lx equal lx
variable Ly equal ly
variable Lz equal lz
variable Vol equal vol
variable thickn equal 2.29
##---------------RELAXATION-----------------------------------
fix 1 all nvt temp 300 300 0.05
thermo 200
run 3000
unfix 1
fix 1 all npt temp 300 300 0.05 x 0 0 0.5 y 0 0 0.5
thermo 2000
run 30000
##---------------DEFORMATION PREPARATION----------------------
unfix 1
reset_timestep 0
fix 1 all nvt temp 300 300 0.05
fix 2 all ave/time 1 100 100 c_2[1] c_2[2]
fix 3 all ave/time 1 100 100 v_Lx v_Ly v_Lz v_Vol
variable srate equal 3.0e9
variable srate1 equal “v_srate / 1.0e12”
fix 4 all deform 1 y erate ${srate1} units box remap x
run 100
##---------------THERMO OUTPUT--------------------------------
variable CorVol equal f_3[4]*v_thickn/(f_3[3])
variable ConvoFac equal 1/1.0e4
variable sigmaxx equal f_2[1]*v_ConvoFac/v_CorVol
variable sigmayy equal f_2[2]*v_ConvoFac/v_CorVol
variable StrainPerTs equal v_srate1*v_ts
variable strain equal v_StrainPerTs*step
thermo 100
thermo_style custom step temp v_strain v_sigmaxx v_sigmayy pe ke lx ly vol
##---------------DUMP FILES-----------------------------------
dump 1 all atom 100 Ti2N_tensile.lammpstrj
dump 2 all custom 100 Ti2N_stress.lammpstrj id type x y z c_1[1] c_1[2] c_1[3] c_1[4] c_1[5] c_1[6]
##---------------STRESS STRAIN OUTPUT-------------------------
fix 5 all print 100 “$(step) {strain} {sigmaxx} ${sigmayy}” file Ti2N_stress_strain.txt screen no title “step strain sigmaxx sigmayy”
##---------------DEFORMATION RUN-------------------------------
run 90000