# Understanding the thermal conductivity calculation in the readme document of KAPPA example

Dear All,

I’m trying to understand the thermal conductivity calculation explained in the readme file of the KAPPA example.

For example the in.ehex explanation (Although the question can be applied to all the other files as well).

(3) in.ehex

``````dQ = (100*100) / 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
xy box area = 18.82^2
divide by 2 since energy flux goes in 2 directions due to periodic z
dTemp = 0.770 from log file for average Temp difference between 2 regions
dZ = 18.82

Kappa = 3.45
``````

When calculating dQ what is the 100 time in tau referring to? it’s not the run time because it’s mentioned later. So what is it? and where is it specified in the in.ehex file?

I attached the in.ehex file and the readme file for later reference.

Best regards,
Yunes Salman

in.ehex (1.6 KB)

It very obviously is.

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I see. Excuse my stupidity.

Dear Axel,

If the

``````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
``````

So my question, is the 100 time in tau and the 100 run time in tau are the same number? if so, why do we multiply and divide by the same run time? It cancels each other.

Thank you.

Best regards,
Yunes Salman

You have to see this in the context of the entire README file. It compares 5 different methods and in most cases you derive kappa from dQ, dTemp, and dZ which in turn are computed with an eye toward what the input and setup for the individual method is. This way you not only can learn how to compute kappa, but also see how the input parameters correlate with the result computation and how they compare between the methods.

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