# Why by changing K12 in pair_coeff command defined in eDPD temperature graph remain the same?

Hello All,

I created a box, in which I have beads type 1 in the left (of y direction) and beads type 2 in the right. I have a heat source on the middle of the right side and a heat sink in the middle of the left region. I am expecting that if I choose thermal conductivity (in the pair_coeff) between region 1 & 2 way smaller that each region’s thermal conductivity, I should see a jump in temperature graph (heat should not conduct between two regions). But what I observe is that on both cases of

K11=k22=k12= 1.5E10^-5 & k11=k22=1.5E10^-5, K12=10^-10

I get the same temperature graph. (beads type1 & beads type2 remain in their region (fix rigid)). Not jump appears in the case of k11=k22=1.5E10^-5, K12=10^-10

Could you please help me understand why I am getting the same graph for both case? Is there anything I am doing wrong?

Here is my code:

units lj
dimension 3
boundary p p p
neighbor 0.2 bin
neigh_modify every 1 delay 0 check yes

atom_style edpd

region edpd block -20 20 -20 20 -10 10 units box
region edpd1 block -20 0 -20 20 -10 10 units box
region edpd2 block 0 20 -20 20 -10 10 units box

create_box 2 edpd #2 is number of atom types

create_atoms 1 random 16000 276439 edpd1
create_atoms 2 random 16000 276438 edpd2

group edpd1 region edpd1
group edpd2 region edpd2

mass 1 1.0
mass 2 1.0

set atom * edpd/temp 1.0

set atom * edpd/cv 1.5E5

pair_style edpd 1.58 9872598

pair_coeff 1 1 18.75 4.5 0.41 1.58 1.45E-5 2.0 1.58

pair_coeff 2 2 18.75 4.5 0.41 1.58 1.45E-5 2.0 1.58

pair_coeff 1 2 18.75 4.5 0.41 1.58 1.45E-10 2.0 1.58

compute mythermo all temp
thermo 100
thermo_modify temp mythermo
thermo_modify flush yes

comm_modify vel yes

velocity all create 1.0 432982 loop local dist gaussian

fix rigid all rigid single

fix mvv all mvv/edpd 0.5
fix upper edpd1 edpd/source cuboid 0 10 0 40 10 20 0.01
fix lower edpd2 edpd/source cuboid 0 -10 0 40 10 20 -0.01

timestep 0.01
run 1000
reset_timestep 0

compute temp all edpd/temp/atom
compute ccT all chunk/atom bin/1d y 0.0 1.0
fix stat all ave/chunk 1 1000 1000 ccT c_temp density/number norm sample file tempK10.profile

run 1000

I just noticed that I am choosing the left region in x direction and right region the positive x derection. I changed my box so that I have two separated region in y direction instead.

region edpd block -20 20 -20 20 -10 10 units box
region edpd1 block -20 20 -20 0 -10 10 units box
region edpd2 block -20 20 0 20 -10 10 units box

but the answer I get for temperature does not make sense at all:

It looks like the thermal conductivities within regions 1 and 2 are high enough that the steady state temperature gradient at the interface between them is negligible. And if there is a negligible steady temperature gradient then there will be a negligible temperature jump across the interface.

Thanks for your response. I tried for much smaller K11 and K22, still does not make sense.
the graph looks like noise.

My suggestion is that you should not worry about getting the “right result” until you understand the result you do have. You should:

• compute profiles of your system in all of the x, y and z directions
• get profiles of mass density as well as temperature, in case you have unexpected inhomogeneities (and visualise your system)
• check that energies and temperatures behave as expected with either one source or both sources turned off
• and any other diagnostics you can think of (I’ve never done a finite-size simulation in LAMMPS so you should know more than me)

All of this will be much faster with fewer simulation particles – I’d suggest halving your system in each direction so you have 1/8 the number of particles to simulate. And keep track of script changes as you go.

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