CO2 self diffusion coefficient

Dear all,
I wanted to determine the self diffusion coefficient of CO2 in supercritical phase. THe slope of msd vs time gives me the value 1.6x10^-8 (m^2/s). According to the literature it should come out of 10^8 order i.e. between 2x10^8 to 3x10^8 (m^2/s). I did minimization then used NVE and then NVE. I achieved the equilibration in 1ns and then did production run for 1 ns. I used real unit with the following lj parameters shown in the input file. Please suggest me what changes should I make to my input script to get the accurate result.

units real
atom_style full
variable T equal 323.2
variable V equal vol
variable dt equal 1

setup problem

dimension 3
boundary p p p
read_data data.1200sco2
pair_style lj/cut/coul/long 12 12
pair_coeff 1 1 0.0573209 2.7918
pair_coeff 2 2 0.164254 3.0
pair_coeff 1 2 0.097 2.9
bond_style harmonic
angle_style harmonic
kspace_style pppm 1.0e-4
bond_coeff 1 2566.68 1.16
angle_coeff 1 295.41 180.0

neighbor 2.0 bin
neigh_modify every 10 delay 20 check yes

group co2 type 1 2

min_style cg
dump 1 all custom 1000 water2_k_MSD_RDF2.lammpstrj id mol type x y z ix iy iz
thermo 1000
thermo_style custom step etotal temp vol press density
minimize 1e-5 1e-7 5000000 10000000

equilibration and thermalization

neighbor 3.0 bin
neigh_modify delay 0 every 1 check yes
neigh_modify exclude molecule co2
velocity all create $T 102486 mom yes rot yes dist gaussian

fix NVE all nve
thermo_style custom step etotal temp vol press density
thermo 1000
timestep ${dt}
run 1000000

unfix NVE

fix NVT all nvt temp $T T 1 timestep {dt}
thermo 1000
dump 2 all custom 1000 cd2000_conductivity.lammpstrj id mol type x y z vx vy vz ix iy iz
run 1000000

compute msd co2 msd com yes
fix msd co2 ave/time 10 100 1000 c_msd[4] file

compute rdf co2 rdf 1000 1 1 1 2 2 2
fix rdf co2 ave/time 10 100 1000 c_rdf file test.rdf mode vector

thermo 1000
dump 3 all xyz 100
timestep ${dt}
thermo_style custom step etotal temp vol press density
run 1000000

What literature says diffusion constants are 10^8 m^2/s? This would imply that in 1 second, a typical gas molecule would diffuse 10 km. A value of 10^-8 m^2/s is in the right ballpark.