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Re: [lammps-users] Young's modulus

# Re: [lammps-users] Young's modulus

 From: "Chowdhury, Sanjib Chandra" Date: Wed, 2 May 2018 21:51:12 +0000

 Depending on strain rate, temp and loading type (plane stress or plane strain) you can get different modulus values.     Are you using the same loading and BCs to get 160 GPa?   Sanjib   From: J Moon Sent: Wednesday, May 2, 2018 5:16 PM To: lammps-users@lists.sourceforge.net Subject: [lammps-users] Young's modulus   Hi,   I am trying to calculate Young's modulus of crystalline silicon.   I am aware that there's the "Elastic" example provided in the package but I wanted to write my own input file using fix deform and compute stress/atom commands and learn how to compute mechanical properties.    I calculated pressure in the x-directon using the compute stress/atom command and used the fix deform command to deform my structure in x direction and recorded the corresponding strain.    Then, I calculated the E = pressure in x/ strain in x and I got ~90 GPa vs ~160 GPa (expected value).   Could you please take a look at my input file and see what could be wrong?   I'd really appreciate your help.   Best,   Jack     # bulk Si via tersoff   units                metal atom_style      atomic   dimension 3 boundary p p p lattice              diamond 5.431 region              box block 0 8 0 8 0 8 create_box      1 box create_atoms   1 box   pair_style        tersoff pair_coeff       * * Si.tersoff Si  mass            1 28.06 group           every region box neighbor        2.0 multi neigh_modify    every 2 delay 4 check yes     ##Simulation Variables variable        temp equal 1         # temperature to be set variable        dt equal 0.0005             # timestep to be used 0.5 fs variable        volume equal 82018.0367          # molecule volume variable           L equal 43.448 #when I used L to be the last lx of the equilibration run 1, it didn't make differences.   ###Do a quick initial run to see initial state of the system   dump initial all atom 1000 ini.atom timestep        \${dt} thermo_style    custom step temp press pe ke etotal vol pxx pyy pzz lx ly lz run             0 undump initial   ###Minimize Energy   min_style       cg minimize        0 1.0e-6 500 5000   ###Equilibration   velocity        all create 1 87625467 rot yes mom yes dist gaussian loop geom      # Create random velocity distribution using random seed reset_timestep  0   ##Equilibration Run 1 dump NPT all atom 50 NPT.atom fix                    3 all npt temp 1 1 0.05 iso 0.0 0.0 0.5 thermo             50 run                   100000 unfix                3 undump NPT   compute peratom all stress/atom NULL #Units are pressure*volume compute fy all reduce sum c_peratom[1] c_peratom[2] c_peratom[3] c_peratom[4] c_peratom[5] c_peratom[6] compute p all reduce sum c_peratom[1] c_peratom[2] c_peratom[3]       #Variables   variable           pressx equal c_fy[1]/vol*1.0e-4 variable           pressy equal c_fy[2]/vol*1.0e-4 variable           pressz equal c_fy[3]/vol*1.0e-4 variable           totalp equal -(c_p[1]+c_p[2]+c_p[3])/(3*vol)*1.0e-4   variable           strainx equal (lx-v_L)/v_L variable           strainy equal (ly-v_L)/v_L variable           strainz equal (lz-v_L)/v_L   variable           tmp      equal temp       fix                    4 all npt temp 1 1 0.05 y 0 0 1000 z 0 0 1000 drag 2 fix                    5 all deform 1 x erate 1e-5 units box remap x fix def1 all print 50 "\${pressx} \${pressy} \${pressz} \${totalp} \${strainx} \${strainy} \${strainz} \${tmp}" file def1.txt screen no dump Deform all atom 100 deform.atom run                   1000000 unfix 4 unfix 5 unfix def1 undump Deform  print "All done"