# Fix Heat; Heat flux calculation;

Dear Lammps users,

I am trying to figure out how to calculated the heat flux imposed on my system using FIX HEAT command.
I was wondering when we set the eflux in the command line, that eflux need to be divided by the timestep as well? I mean when I read the lammps manual on this topic it says: “eflux determines the change in aggregate energy of the entire group of atoms per unit time”
It gets me confused, And I hear different things from my peers, some of them believe I need to divide it by the timestep value and some say no.

Thanks,

Best,
Rouzbeh

Dear Lammps users,

I am trying to figure out how to calculated the heat flux imposed on my
system using FIX HEAT command.
I was wondering when we set the eflux in the command line, that eflux need
to be divided by the timestep as well? I mean when I read the lammps manual
on this topic it says: "eflux determines the change in aggregate energy of
the entire group of atoms per unit time"

i don't think that this is confusing, but rather to the point. but
that is just me.

It gets me confused, And I hear different things from my peers, some of them
believe I need to divide it by the timestep value and some say no.

rather than just giving you a yes/no answer, it would be much better
for you to convince yourself what is the right thing. so how about you
have a look at the example(s) provided by LAMMPS in the examples/KAPPA
folder? and then check which of the two options applies?

axel.

Thank you,
I have been looking through the examples, but still not sure about some the values that are used in the example.

in the example the part of the script that is used to calculate the KAPPA is as following:

"
fix hot all heat 1 100.0 region hot
fix cold all heat 1 -100.0 region cold

fix 2 all ave/spatial 10 100 1000 z lower 0.05 v_temp &
file profile.heat units reduced

run 20000

"

And the way is explained is :

"dQ = (100100) / 100 / 18.82^2 / 2
100
100 = 100 (time in tau) * 100 (energy delta specified in fix heat)
100 = 20,000 steps at 0.005 tau timestep = run time in tau
"
But still I am not sure what is the variable “TIME IN TAU”.

I truly appreciate it if you could help me on this issue. We are having a huge conflict figuring it out, And your help will be greatly appreciated.

Sincerely,
Rouzbeh

tau is the Lennard Jones unit of time

If the timestep is 0.005 then there are 200 timesteps for one tau

The run is 20,000 timesteps or 100 tau

Steve