calculation of temperature/kinE LAMMPS/LIGGGHTS

Submitted by Silias on Mon, 06/06/2011 - 16:11

Hi,

actually I tried to figure out in the LAMMPS manual, how I could access the kinetic energy of my granular system.
I understood, that LAMMPS is calculating the kinetic energy in a MD sence (ke = 1/2 Kb T).

I wonder if the ke in my thermo-output is calculated like 1/2mv^2 for all particles if I use the LIGGGHTS package?

Thanks in advance,

Sebastian

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ckloss's picture

ckloss | Mon, 06/06/2011 - 16:21

Hi Sebastian,

you are right, but nevertheless the ke thermo keyword indeed calculates the correct kinetic energy, because 3/2 kb T _is_ equal to the kinetic energy. You can check this by

compute 2 all ke/atom
compute 3 all reduce sum c_2
thermo_style custom step atoms ke c_3

(compute ke/atom uses m v^2/2 - you can check that in the source code)
The only thing that might be confusing about this is that the temperature used for this calculation does not have a meaning in a macroscopic sense, but "only" in an MD sense

Christoph

Silias | Mon, 06/06/2011 - 16:56

Hi Christoph,

thanks for your fast reply!
ok, wow that's surprising...

just for better understanding:

if both formulas calculate the same kinetic energy then, with

1/2 · m_particle · v_particle^2 = 3/2 · Kb · T

the temperature for 1 particel with mass=1kg and velocity=1m/s the Temperature of that particle would be
2.41 · 10^22 Kelvin (with Kb = 1,3806504 · 10−23 J/K).

So, if I would do a MD simulation this hypothetical temperature would be the temperature of my atoms, but as I am doing simulations with granular materials I can forget about that value!?

Thanks

Sebastian

ckloss's picture

ckloss | Mon, 06/06/2011 - 17:58

>>I can forget about that value!?
Well, depending on the circumstances, this value will make sense (in the sense of statistical mechanics) or not.
For one particles it won't - for other situations it will. You may want to google "kinetic theory of granular gases" for details

Christoph