# Pressure values for liquid Ar L-J interaction

Dear lammps users,

I am trying to estimate the pressure values for liquid Ar at a density of 1.3954 g/cc. This has been done by putting 2048 atoms of Ar in 46A cubic box.

Number of atoms = 2048
Mass of Argon = 39.948 gm/mole
Na = 6.022*10^23
Box Length = 46 A
angstrom to cm = 10^-8

density = (Number of atoms)*(Mass of Argon/Na)/(Box Length *angstrom to cm)^3
Putting in all the variables from above, we get,

density = 2048*(39.948/(6.02210^23))/((4610^-8)^3)
density = 1.3954 gm/cc

where they specify value of epsilon/Kb = 125.7 K and sigma = 0.3345 nm. These values when converted to corresponding lammps real units become epsilon = 250 and sigma = 3.34 A.

The cutoff in units real then becomes 2.5*sigma which is 8.34 A.

So my pair coefficient are pair_coeff * * 250 3.34 8.34

The value of pressure at 47 K should be close to 10 Pa, but I am getting -10^7 atm.

I do not know what error I am committing here. Is units real option a problem here Should I do the calculation in units LJ.

I am attaching my data file and input file along with this email. I am using Nov, 2016 version of the lammps stable release.

P.S. I am using units real and hence my coordinates are in angstrom.

Thanks,
Ankit

data.ar_2048 (91.5 KB)

in.test (1.38 KB)

Dear lammps users,

I am trying to estimate the pressure values for liquid Ar at a density of
1.3954 g/cc. This has been done by putting 2048 atoms of Ar in 46A cubic
box.

Number of atoms = 2048
Mass of Argon = 39.948 gm/mole
Na = 6.022*10^23
Box Length = 46 A
angstrom to cm = 10^-8

density = (Number of atoms)*(Mass of Argon/Na)/(Box Length *angstrom to
cm)^3
Putting in all the variables from above, we get,

density = 2048*(39.948/(6.022*10^23))/((46*10^-8)^3)
density = 1.3954 gm/cc

I am using the L-J parameters by "John A. White "Lennard-Jones as a model
for argon and test of extended renormalization group calculations", Journal
of Chemical Physics 111 pp. 9352-9356 (1999"

where they specify value of epsilon/Kb = 125.7 K and sigma = 0.3345 nm.
These values when converted to corresponding lammps real units become
epsilon = 250 and sigma = 3.34 A.

an epsilon of 250 kcal/mol seems off by a factor of 1000. typical
epsilon values for real units are just under 1kcal/mol.

The cutoff in units real then becomes 2.5*sigma which is 8.34 A.

please note, that you need to converge the cutoff or adjust the
pressure for contributions beyond the cutoff, if you want accurate
pressure values.

axel.