# Which one to use (Stress/atom or Pressure ) to evaluate the elastic modulus of CNT

Dear all
I am trying to draw the stress strain curve for cnt under axial strain to calculate the elastic modulus of CNT. I tried with the online available examples given to plot the stess-strain curve but it was using pressure command and results are not coming whereas while using the stress/atom command multiplied by volume of CNT is giving relatively closer results. I am stuck with it because for further exploration I need to find a reliable way to predict the elastic modulus. I am writing my code here, any help will be deeply appreciated.

#initialization

units metal
dimension 3
boundary p p p
atom_style atomic

#intermoleculr force

pair_style airebo 3.0 1 1
pair_coeff * * /home/ashish/lammps-10Feb15/potentials/CH.airebo C

#---------------Energy Equilibriation--------------------------
reset_timestep 0
timestep 0.001
velocity all create 300 12345 mom yes rot no
fix 1 all npt temp 300 300 1 iso 0 0 1 drag 1

# Set thermo output

thermo 1000
thermo_style custom step lx ly lz press pxx pyy pzz pe temp

# Run for at least 10 picosecond (assuming 1 fs timestep)

run 1000
unfix 1

#--------------------Storing Initial length---------------------
variable tmp equal “lz”
variable L0 equal {tmp} print "Initial Length, L0: {L0}"

#------------Deform------------------------------
reset_timestep 0
fix 1 all npt temp 300 300 1 x 0 0 1 y 0 0 1 drag 1
fix 2 all deform 1 z erate 0.001 units box

variable strain equal “(lz - v_L0)/v_L0”
variable p1 equal “v_strain”
variable p2 equal “-pxx/10000”
variable p3 equal “-pyy/10000”
variable p4 equal “-pzz/10000”

# ------------------------- SETTINGS ---------------------------------

#### Computes Required

compute csym all centro/atom 12
compute 2 all stress/atom NULL
compute mytemp all temp
compute 11 all reduce sum c_2[1]
compute 12 all reduce sum c_2[2]
compute 13 all reduce sum c_2[3]
compute 14 all reduce sum c_2[4]
compute 15 all reduce sum c_2[5]
compute 16 all reduce sum c_2[6]

#variable Ez equal “v_p4/v_strain” # using pressure

variable volume equal 3.148\${L0}*3.4

variable sigma equal (c_13)/(10^4*v_volume) # using stress/atom
variable Ez equal v_sigma/v_p1

log /home/ashish/Desktop/cnt.txt

# Display thermo

thermo 1000
thermo_style custom step temp press lz v_p2 v_p3 v_p4 v_strain v_Ez v_sigma

run 30000

ashish srivastava
MNIT Jaipur

As the compute stress/atom doc page explains,

you should get identical values if you sum stress/atom

and normalize by the apporpriate volume as compared

to the global compute pressure. So there is no real

difference between the two.

Steve

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