[lammps-users] What is the definition of the kinetic energy ?

Dear all.

I cannot obtain the kinetic energy that I expected.

To check how to print the energy, I made simple program.
The program describes the uniform motion for 1 particle.
Then, the trajectory of position become parabola,
and the velocity uniformly increases.

For this case, I expect that the kinetic energy
also increases because the velocity increases.

However, the result is not so.
The kinetic energy obtained by "thermo_style custom ke"
keep 0 even when the velocity is not zero.


The input script I used is given below:

echo log
units lj
atom_style charge

pair_style none
#read_data input.lammps

lattice sc 1
region box block 0 1 0 1 0 1
create_box 1 box
create_atoms 1 single 0.0 0.0 0.0
mass 1 1.0

velocity all zero linear

timestep 0.01

fix 2 all nve

group molset type 1
set group molset charge 1

#fix 5 molset efield 0.1 0.0 0.0
fix 5 molset addforce 0.1 0.0 0.0

# output thermodynamic data
thermo_style custom step etotal pe ke press
thermo_modify norm no
thermo 10

dump dump3 molset custom 1 config.dat x vx

run 1000

KE is derived from temperature which subtracts
degrees of freedom for a large ensemble. See
the compute_modify extra 0 command to avoid this.
Or just compute the KE of one atom yourself
with a variable = 0.5mv^2 or see the compute ke/atom


Dear Damien and Steve Plimpton

Thank you for the response.
I used the compute_modify extra 0 command.
And I obtain what I want.

Best regards.

Takenobu Nakamura