Why do we have a negative stress value at the initiation of the uniaxial loading

Hello all,

I am trying to make a uniaxial loading of a Fe-Mn binary system, however, when I started to deform my simulation box, the stress appeared to be negative.

Here is my input script

LAMMPS Input File for Deformation Induced Martensitic Transformation

######################################

SYSTEM INITIALIZATION

clear
units metal
dimension 3
boundary p p p
atom_style atomic

INTERATOMIC POTENTIALS

pair_style meam/c
pair_coeff * * library.meam Fe Mn FeMn-para.meam Fe Mn

SIMULATION SETTINGS

compute csym all centro/atom fcc
compute eng all pe/atom
compute eatoms all reduce sum c_eng

######################################

VARIABLE DEFINE

variable n_iter equal 20000

variable srate equal 1.0e10
variable srate1 equal “v_srate / 1.0e12”

MINIMIZATION 1 (UNDER 0K)

reset_timestep 0
fix 1 all box/relax iso 0.0 vmax 0.001
thermo 100
thermo_style custom step pe lx ly lz press pxx pyy pzz c_eatoms

dump_modify 1 element Fe Mn

min_style cg
minimize 1e-18 1e-18 10000 10000
unfix 1

MINIMIZATION 2 (UNDER 300K)

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

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

run \${n_iter}
unfix 1

Store final cell length for strain calculations

variable tmp equal “lx”
variable L0 equal {tmp} print "Initial Length, L0: {L0}"

UNIAXIAL DEFORMATION

reset_timestep 0
fix 1 all npt temp 300 300 1 y 0 0 1 z 0 0 1 drag 1
fix 2 all deform 1 x erate \${srate1} units box remap x

p2, p3, p4 are in GPa

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 Fe_5Mn_tens.def1.txt screen no

DUMP THE RESULTS

dump 1 all custom 1000 Fe_5Mn_tensile.dump mass type xs ys zs c_csym c_eng fx fy fz
dump_modify 1 element Fe Mn

DISPLAY THERMO

thermo 1000
thermo_style custom step v_strain temp v_p2 v_p3 v_p4 ke pe press
run 20000

ALL DONE

unfix 1
unfix 2
undump 1
print “ALL DONE”

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