Dear all, my system is a mixture of 3 organic components and I am trying to calculate the thermal conductivity using green-kubo method. I find that at high temperatures the integrals in the LAMMPS output converges, but they do not at low temperatures of 300 and 358K. Two of these integrals would usually converge but the third will be so way of and I would get a k value of about 0.4~0.5 W/mK whereas experimental result is 0.19W/mK. I have tried a lot of things based on the mailing list, but no solution so far.

I have attached a part of my input file here and would appreciate your help. I use the OPLS-AA FF if that helps. Thank you.

variable T equal 300 # Simulated Temperature [K]

variable p equal 10000 # correlation length

variable s equal 1 # sample interval

variable d equal $p*$s*20 # dump interval

variable V equal vol

variable dt equal 1.0 # timestep

# Setup Parameter

units real

dimension 3

boundary p p p

atom_style full

neighbor 2.0 bin

neigh_modify every 1 delay 0 check yes page 1000000

kspace_style pppm 1.0e-6

pair_style lj/charmm/coul/long 10.00 10.10

bond_style harmonic

angle_style harmonic

dihedral_style harmonic

improper_style cvff

atom_modify sort 0 0.0

read_restart file-300-2.lammps-run08

timestep ${dt}

velocity all create $T 102486 dist gaussian

reset_timestep 0

compute myKE all ke/atom

compute myPE all pe/atom

compute myStress all stress/atom NULL virial

compute flux all heat/flux myKE myPE myStress

variable Jx equal c_flux[1]/vol

variable Jy equal c_flux[2]/vol

variable Jz equal c_flux[3]/vol

fix 1 all nve

fix JJ all ave/correlate $s $p d &
c_flux[1] c_flux[2] c_flux[3] type auto file profile.dat ave running
variable scale equal {convert}/${kB}/$T/$T/$V*s*{dt}

variable k11 equal trap(f_JJ[3])*{scale}
variable k22 equal trap(f_JJ[4])*{scale}
variable k33 equal trap(f_JJ[5])*${scale}

thermo_style custom step temp pe etotal press vol v_Jx v_Jy v_Jz v_k11 v_k22 v_k33

thermo $d

# Backup and total number of steps

restart 5000 {name}-{tempname}-1.lammps-run${runnumber} {name}-{tempname}-2.lammps-run${runnumber}

run 5000000

variable k equal (v_k11+v_k22+v_k33)/3.0

variable ndens equal count(all)/vol

print “average conductivity: $k[W/mK] @ T K, {ndens} /A^3”