Hi All,

I am trying to conduct this kind of simulation for getting stress-strain relationship for copper nanowire. But, I run this code, then I used to getting an error as’ Error on process 0, substitution for illegal variable P. Would you like to help me in solving this problem? I have involved with solving this problem for 3 days, still the case is same, and couldn’t identify what is the problem happening here. The LAMMPS version April 8, 2021.

```
#Initialization
units metal
dimension 3
boundary p p p
#Atom and Lattice Definition
atom_style atomic
#definition of type of lattice
lattice fcc 3.52
# Definition of Region
region nanowire block 0 40 0 40 0 40
create_box 3 nanowire
create_atoms 1 region nanowire
mass 1 63.546
variable EngVol equal 10^4*(29.496)*(102.177144)*(102.7026)
variable T equal 0.01
#Variable P equal 0
#Forcefields
pair_style eam
pair_coeff * * Cu_u3.eam
#Definition of the region to set velocity in upper and lower side
variable L equal 144.6
variable StRate equal 0.0001
####Minimization
min_style cg
minimize 1e-25 1e-25 100000 100000
timestep 0.001
#Give some initial velocity to all the atoms randomly at a temperature of 0.01K.
velocity all create 0.01 511124 rot yes mom yes
fix 0 all npt temp $T $T 0.1 iso $P $P 1 #tdamp is 100 times of timestep (0.001*100)
run 1000
unfix 0
###############deformation
reset_timestep 0
fix 1 all npt temp $T $T 0.1 x 0 0 1 z 0 0 1 ###to deform in y direction
###### compute
compute stN11 all stress/atom NULL
compute stN12 all reduce sum c_stN11[1]
compute stN13 all reduce sum c_stN11[2]
compute stN14 all reduce sum c_stN11[3]
variable strain equal (ly-v_L)/v_L
variable s1 equal "v_strain"
variable Sgiga1 equal (c_stN12)/(${EngVol})
variable Sgiga13 equal (c_stN13)/(${EngVol})
variable Sgiga14 equal (c_stN14)/(${EngVol})
fix 2 all deform 1 y erate ${StRate} units box
#Simulation settings
dump 1Dump all atom 100 dump.atom
thermo 100
thermo_style custom step lx ly lz xlo xhi ylo yhi zlo zhi vol temp etotal press v_strain pxx pyy pzz
fix def1 all print 100 "${s1} ${Sgiga1} ${Sgiga13} ${Sgiga14}" file stress_strain.txt screen no
#Run
run 5000
write_data nanotube.data
```

Sincerely,

Akash Talapatra