Hi lammps-user,

i am simulating bulk silicon with tempreture (300 K), static boundry condition, Tersoff potential. I choose 5.445 A for the lattice constant (Ref: “Molecular dyanmycs simulation of bulk silicon under strain” by H. Zhao and N. R. Aluru). I would like to apply compressive load at that tempreture. But the problem is: the pressure (press at thermo_style) dosnt relax and it is changing even after 1 nanosecond (1melion steps) of simulation time. Maybe i dont know the correct “drag” factor in “fix” command!!. Could you please tell me what i am doing wrong?


The input file is:

#Si geometry creation, relaxation and compressive loading

#set dimension and boundry condition
dimension 3
boundary s s s

#set units type,atom type and neigborhood
units metal
atom_style atomic
neigh_modify every 1
#create big box
lattice diamond 5.445
region big block -12 12 -12 12 -12 12
create_box 1 big
#create main atoms
lattice diamond 5.445
region main block -1.5 1.5 -1.5 1.5 -1.5 1.5
create_atoms 1 region main

#create sefr region
region sefrsefr block -1.5 -1.4 -1.5 1.5 -1.5 1.5
group sefr region sefrsefr

#create bist region
region bistbist block 1.4 1.5 -1.5 1.5 -1.5 1.5
group bist region bistbist

#create center17 region
region centercenter17 block -1.3 1.3 -1.5 1.5 -1.5 1.5
group center17 region centercenter17

#set the mass of atoms
mass 1 28.0855

#set the velocity of atoms
velocity all create 300.0 4928459 rot yes mom yes

#set the velocity of atoms of left and right to zero
velocity sefr set 0.0 0.0 0.0
velocity bist set 0.0 0.0 0.0

#set forces to sefr and bist
fix 144 sefr setforce 0.0 0.0 0.0
fix 155 bist setforce 0.0 0.0 0.0

#define the tersoff potential
pair_style tersoff
pair_coeff * * Si.tersoff Si

#set timestep
timestep 0.001

#compute stress per atom and potential per atom
compute stress all stress/atom
compute poten all pe/atom

compute stresscenter17x all reduce/region centercenter17 sum c_stress[1]
compute stresscenter17y all reduce/region centercenter17 sum c_stress[2]
compute stresscenter17z all reduce/region centercenter17 sum c_stress[3]

dump 101 all xyz 100000
dump 117 center17 xyz 100000
dump 17 center17 custom 100000 dump.center17 x y z c_stress[1] c_stress[2] c_stress[3] c_stress[4] c_stress[5] c_stress[6] c_poten

#specify values to show in run
thermo 10000
thermo_style custom step temp pe ke etotal press c_stresscenter17x c_stresscenter17y C_stresscenter17z lx ly lz

#specify the method of calculation
fix 201 all nvt 300.0 300.0 0.001 drag 0.5 chain no
run 1000000

#strain load 1
fix 201 all nvt 300.0 300.0 0.001 drag 0.5 chain no
displace_atoms bist move -0.01 0 0 units box
run 100000

#strain load 1
fix 201 all nvt 300.0 300.0 0.001 drag 0.5 chain no
displace_atoms bist move -0.01 0 0 units box
run 100000 & nbsp;


You’re using fix nvt so you’re not controlling the pressure
at all. It is what it is, and pressures fluctuate. However you
are using 0.001 for your damping constant in fix nvt which
is far too small. That is equiv to one timestep in metal
units. Read the fix nvt doc page and use something more
like 0.1.