[lammps-users] Problems in thermal conductivity calculation

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1

Dear LAMMPS users,
  I am now using "fix thermal/conductivity" command to calculate thermal
conductivity for my system. However, the temperature profile between the hot and
cold slabs does not seem to change with time. I tried to increase Nswap to 200
steps, but it did not work.
  The followings are my input scripts and output temperature profile. I would be
appreciate if someone can give me some advices.

Sincerely,

Keng-Hua Lin

========================input script====================================

#Si
units real
atom_style atomic
boundary p p p
lattice diamond 5.43 orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
region box block 0 1 0 1 0 1 units lattice
create_box 1 box
create_atoms 1 box
mass 1 28.0855

replicate 3 3 10

pair_style sw

velocity all create 300.0 82373 dist gaussian

pair_coeff * * SiGe.sw Si #can also be defined in datafile

timestep 1.00000000
thermo 50

#---------------flux calculation -------------------------------------

compute ke all ke/atom
variable temp atom c_ke*1000/(1.5*1.98721)
fix 3 all ave/spatial 1 5000 10000 z lower 0.1 v_temp file tmp.profile
units reduced

fix 2 all thermal/conductivity 50 z 10
variable DeltaPe equal f_2
thermo_style custom etotal ke pe temp press vol pxx pyy pzz pxy pxz pyz lx
ly lz v_DeltaPe
thermo_modify flush yes line multi
log NPT300K_DeltaPe_BulkSi.log

run 100000

========================temperature profile===============================

# Spatial-averaged data for fix 3 and group all
# Timestep Number-of-layers
# Layer Coord Ncount v_temp
10000 10
  1 0.05 72 132.035
  2 0.15 72 314.859
  3 0.25 72 262.382
  4 0.35 72 291.567
  5 0.45 72 293.415
  6 0.55 72 518.897
  7 0.65 72 307.96
  8 0.75 72 291.909
  9 0.85 72 290.409
  10 0.95 72 292.393
20000 10
  1 0.05 72 132.035
  2 0.15 72 314.859
  3 0.25 72 262.382
  4 0.35 72 291.567
  5 0.45 72 293.415
  6 0.55 72 518.897
  7 0.65 72 307.96
  8 0.75 72 291.909
  9 0.85 72 290.409
  10 0.95 72 292.393
.....................
.....................
.....................

100000 10
  1 0.05 72 132.035
  2 0.15 72 314.859
  3 0.25 72 262.382
  4 0.35 72 291.567
  5 0.45 72 293.415
  6 0.55 72 518.897
  7 0.65 72 307.96
  8 0.75 72 291.909
  9 0.85 72 290.409
  10 0.95 72 292.393

2

Try decreasing N (e.g. to 1) if you want to
get a more dramatic profile change. So it is
swapping every step.

Steve

3

I am not surprized at your result. In your input file, I did not see any fix with updates the coordinates and velocities of the atoms. So, same velocities, same temperature of each timestep and hence same average

4

Hi, Vikas,
  Thank you for your reply. I added nvt command into my input script and the
problem has been solved. Thank you again.

Keng-Hua Lin

Quoting Vikas Varshney <[email protected]...>:

5

Using fix nvt with fix thermal/conducitivity seems like a bad idea.
You won't get a meaningful temperature profile across the
region between the t/c swapping if you are thermosttating it
with another command.

Steve