[lammps-users] Melting transition for Argon

Hi,
I wish to simulate Argon using LJ potential in order to determine its melting transition temperature. From the literature, the melting temperature for Argon is 83.8K. After the simulation, I plotted a graph of volume vs temperature from the output. Unfortunately, I see no abrupt change in the volume at temperature ~83K. Is there any mistake in my input file? Thank you.

LAMMPS melt_argon

units real
boundary p p p
atom_style atomic

dimension 3
lattice fcc 5.260
region box block 0 10 0 10 0 10

create_box 1 box
create_atoms 1 box

pair_style lj/cut 3.5
pair_coeff 1 1 0.238067 3.405

group Ar type 1
mass 1 39.948

timestep 0.001
thermo 1000
neighbor 0.6 bin
neigh_modify every 1 delay 0 check no

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

compute 3 all pe/atom
compute 4 all ke/atom
compute 5 all coord/atom 3.0
compute 6 all temp

velocity all create 0.01 112387 dist gaussian
fix extra all print 100 “{N} {T} {V} {Press} {kine} {pote} ${Etotal}” file data

fix 1 all nvt temp 0.01 0.01 1.0 drag 0.2
run 20000
unfix 1

fix 1 all npt temp 0.01 300.0 10.0 iso 1.0 1.0 1.0
run 500000

Calculating a melting temperature accurately is more
complicated than just running NPT with a changing
temperature. I suggest you read some MD books
and papers that discuss it. One thing to do is detect
when the system goes from ordered (solid) to disordered
(liquid), e.g. by computing a structure factor. One difficulty
is nucleating disorder in a perfect crystal. Another
method is to model a solid/liquid interface and see
which way it moves. When you're at Tmelt it doesn't move.

Steve