Dear Agraj Abhishek,

Please include this mail in lammps forum so that everyone can be benefit.

See below for the input script which you requested. I take the simulation box boundary lx (from thermo_style) as the lattice constant. I wrote a simple code to read lammps output and calculate the lattice constants by averaging over large equilibrium time steps. I believe you could do it too. I don’t understand what you mean by U shape curve, maybe you can post your query to lammps forum.

Good luck,

Christopher

```
# strontium titanate system
atom_style charge
units real
dimension 3
boundary p p p
# lattice
lattice custom 3.9051 a1 1.0 0.0 0.0 a2 0.0 1.0 0.0 a3 0.0 0.0 1.0 basis 0.0 0.0 0.0 basis 0.5 0.5 0.5 basis 0.5 0.5 0.0 basis 0.5 0.0 0.5 basis 0.0 0.5 0.5
region simbox block 0 9 0 9 0 9
create_box 3 simbox
create_atoms 1 box basis 1 1 basis 2 2 basis 3 3 basis 4 3 basis 5 3
group sr type 1
group ti type 2
group o type 3
mass 1 87.62
mass 2 47.867
mass 3 15.999
set group sr charge 1.2
set group ti charge 2.4
set group o charge -1.2
# force-field
pair_style hybrid/overlay born 11.0 coul/long 11.0 morse 11.0
pair_coeff * * coul/long
pair_coeff 1 1 born 0.32 0.32 2.396 100.0 0.0
pair_coeff 1 2 born 0.34 0.34 2.253 250.0 0.0
pair_coeff 1 3 born 0.32 0.32 3.124 200.0 0.0
pair_coeff 2 2 born 0.36 0.36 2.110 625.0 0.0
pair_coeff 2 3 born 0.34 0.34 2.981 500.0 0.0
pair_coeff 3 3 born 0.32 0.32 3.852 400.0 0.0
pair_coeff 1 3 morse 2.41 1.18 2.7615
pair_coeff 2 3 morse 4.23 3.82 2.1923
kspace_style ewald 0.0001
pair_modify tail yes shift no
# initialize
timestep 1.0
velocity all create 298.0 1234567 mom yes rot yes dist gaussian
neighbor 4.0 bin
neigh_modify every 1 delay 0 check yes
# output
variable N equal step
variable pote equal pe
variable Etotal equal etotal
variable T equal temp
variable Press equal press
variable V equal vol
variable kine equal ke
variable Lx equal lx/9
variable Ly equal ly/9
variable Lz equal lz/9
fix extra all print 1 "${N} ${T} ${V} ${Press} ${kine} ${pote} ${Etotal}" file strontium_titanate.out
fix cell all print 1 "${N} ${T} ${Lx} ${Ly} ${Lz}" file strontium_titanate.cell
dump snapshot all custom 1000 strontium_titanate.lammpstrj id q type x y z
# equilibration & thermalization
fix NPT all npt temp 298.0 298.0 100.0 iso 1.01325 1.01325 1000.0
run 220000
```