[lammps-users] Is there heat current leak on fixed boundary, when set the boundary atoms velocity to zero.

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1

Dear all,

I tried to calculate thermal conductivity of single layer graphene. Part of my input script is listed at the end of this email.

In this script, I use fix…setforce 0.0 … and velocity lend set 0.0… commands to make atoms at the two ends of this graphene still. Thus temperature in this region will be 0K. But the rest part of this graphene are around 300K. Will there be large heat current flowing to lend and rend? If so, what can I do to make them adiabatic boundary? Thank you!

lend and rend are left and right end of the graphene to be fixed still.

region 1 block INF -8.8 INF INF INF INF units box
group lend region 1
region 2 block 58.9 INF INF INF INF INF units box
group rend region 2
region 3 block -4.4 5.5 INF INF INF INF units box
group cold region 3
region 4 block 44.6 54.4 INF INF INF INF units box
group hot region 4

…

velocity all create 300.0 872879 dist gaussian mom yes

fix 1 lend setforce 0.0 0.0 0.0
fix 2 rend setforce 0.0 0.0 0.0
velocity lend set 0.0 0.0 0.0 units box
velocity rend set 0.0 0.0 0.0 units box

fix 3 all nvt temp 300.0 300.0 0.2 drag 2.0
timestep 0.0005
run 800000

unfix 3

fix 5 all nve
fix 6 cold temp/rescale 1 150.0 150.0 0.01 1.0
fix 7 hot temp/rescale 1 300.0 300.0 0.01 1.0

Best regards,

Yan

2

If you simply set the force and velocity of some atoms on a boundary
to 0.0, you're not conserving anything or monitoring the flow
of anything. You're just turning off effects that would otherwise
be present. I suggest you look at the commands in LAMMPS
designed to calcluate heat fluxes: compute heat/flux and
fix thermal/conductivity and read the associated papers.

Steve