ke

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1

Dear lammps users：
Who can help me to answer some questions:
In manual, we know that we can get ke from the comand thermo_style(name ke1 ), and we can also get the ke from the comand compute ke/atom(name ke2 ).
Question:
Is ke1 kinetic energy of system ? from this formula ke1=3/2KT ?
Is ke2 kinetic energy of atom?
What’s the relationship of k1 and k2?
Do the sum of kinetic energy of all atoms equal ke1? but ke1 is a tensor.
Thanks
Jack zhuang

2

From the compute ke doc page, “There is a subtle difference between the quantity calculated by this compute and the kinetic energy calculated by the ke or etotal keyword used in thermodynamic output, as specified by the thermo_stylecommand. For this compute, kinetic energy is “translational” kinetic energy, calculated by the simple formula above. For thermodynamic output, the ke keyword infers kinetic energy from the temperature of the system with 1/2 Kb T of energy for each degree of freedom. For the default temperature computation via the compute temp command, these are the same. But different computes that calculate temperature can subtract out different non-thermal components of velocity and/or include different degrees of freedom (translational, rotational, etc).”.

compute ke is the sum of compute ke/atom over all atoms. When you sum this up, compute ke also gives a scalar.

Ray

3

Dear lammps users：

Who can help me to answer some questions:
We know that we can get ke from the command compute ke/atom, and also we can get pe from the command compute pe/atom.
So my question:

  1. how to get the energy/atom from lammps?simplely ke/atom + pe/atom? but it is a tensor, how to output it?
  2. and how to get the instantaneous mean energy of all particles ?

Thanks
Jack zhuang

4

Dear lammps users：
      Who can help me to answer some questions:
      We know that we can get ke from the command compute ke/atom, and
also we can get pe from the command compute pe/atom.
So my question:
1. how to get the energy/atom from lammps?simplely ke/atom + pe/atom?
but it is a tensor, how to output it?

From thermodynamics internal energy is PE + KE. What energy were you

thinking about?

See this section http://lammps.sandia.gov/doc/Section_howto.html#howto_15 on
how to output ke/atom and pe/atom. ke/atom and pe/atom calculates per-atom
vectors, not tensors.

2. and how to get the instantaneous mean energy of all particles <e>?

Take the average? See "thermo_modify" command "norm" keyword for more info.

Ray