thermal conductivity calculation problem

Hi, guys,

I found the Argon thermal conductivity values are accumulative and linear instead of constant value. The results are bothering me a few days. I could not figure out what is wrong place.

The conclutions i got are: I am sure my simulation reach equilibrium; And the temp profile are linear based on bins; the heat flux profile is linear; The temp gradient is constant.

I post my input file here:(temp profile,heat flux file and temp gradient are in the attchment)

NEMD simulation of Ar thermal conductivity


units lj

dimension 3

newton on

boundary p p p

atom_style atomic

neighbor 0.3 bin

neigh_modify check yes

lattice fcc 0.844

region box block -4 4 -4 4 -4 4 units lattice

create_box 1 box

create_atoms 1 box

region hot block INF INF INF INF -0.5 0 units lattice

region cold block INF INF INF INF 3.5 4 units lattice

mass 1 1.0

velocity all create 0.71 458127641 mom yes rot yes dist gaussian units box

pair_style lj/cut 2.8

pair_coeff 1 1 1.0 1.0 # LJ parameters for Ar-Ar

fix temp all temp/berendsen 0.71 0.71 0.05

fix nve all nve

compute ke all ke/atom

variable temp atom c_ke/(1.5*1.0)

fix temp_profile all ave/spatial 1 100000 100000 z lower 0.5 v_temp file temp.profile units lattice

compute hot_temp all temp/region hot

compute cold_temp all temp/region cold

variable delta_temp equal c_hot_temp-c_cold_temp

fix delta_out all ave/time 1 100000 100000 v_delta_temp file delta_temp.dat -------------# temperature gradient

thermo_style custom step temp etotal vol

thermo_modify lost warn

thermo 100


timestep 0.005

run 100001

unfix temp

fix heat_swap all thermal/conductivity 10 z 16

fix e_exchange all ave/time 10 10000 100000 f_heat_swap file e_exchange.dat -----------#Energy exchanged

variable thermal_conductivity equal f_e_exchange/(0.00510.02.0*f_delta_out)1.3806504e-23/(3.405e-103.405e-10)*sqrt(1.67e-21/6.633e-26) ---------# lj units to real units

fix thermal_conductivity_out all ave/time 100000 1 100000 v_thermal_conductivity file thermal_conductivity.dat


run 10000000

Hopefully, somebody can give me some help. Thanks in advance!



Thermocond_Ar.docx (20.5 KB)

If your temperature profile is static (equilibrated system),
then the thermal conductiving has to just be a value, and
not changing with time. The kappa is just the ratio
of the temperature gradient to the energy flux you
are putting in. Flux is energy/time so it is a constant
as well. Fix thermal/cond is accumulating the cummulative
energy it has exchanged, but you have to divide that
by time, and it will thus be a constant.