# Kinetic energy vs Temperature

Hi there,

I am trying to simulate a 2D system of N particles in p-T ensemble.

I am measuring kinetic energy (K) and temperature (T) and printing them through the following rows:

compute 3 all ke
compute 4 all temp
variable N equal atoms
variable kin equal c_3
variable tem equal c_4
fix PRINT all print 1000 “{N} {kin} \${tem}” append xcm.dat

Problem is: plotting the ratio K/T i find N-1 instead of N.

On the instructions I find instead [http://lammps.sandia.gov/doc/compute_temp.html]

«The temperature is calculated by the formula KE = dim/2 N k T, where KE = total kinetic energy of the group of atoms (sum of 1/2 m v^2), dim = 2 or 3 = dimensionality of the simulation, N = number of atoms in the group, k = Boltzmann constant, and T = temperature.»

I guess that, since units are lj , k_B=1, yet I cannot understand why K/T=N-1 rather than N.

Regards

Mario

ps I attach the code to reproduce what I am saying. In xcm.dat you will find K as the second column, T as the third one.

in.pour (1.28 KB)

Hi there,

I am trying to simulate a 2D system of N particles in p-T ensemble.

I am measuring kinetic energy (K) and temperature (T) and printing them
through the following rows:

compute 3 all ke
compute 4 all temp
variable N equal atoms
variable kin equal c_3
variable tem equal c_4
fix PRINT all print 1000 "\{N\} {kin} \${tem}" append xcm.dat

Problem is: plotting the ratio K/T i find N-1 instead of N.

On the instructions I find instead
[Request Rejected]

«The temperature is calculated by the formula KE = dim/2 N k T, where KE =
total kinetic energy of the group of atoms (sum of 1/2 m v^2), dim = 2 or 3
= dimensionality of the simulation, N = number of atoms in the group, k =
Boltzmann constant, and T = temperature.»

I guess that, since units are lj , k_B=1, yet I cannot understand why
K/T=N-1 rather than N.

because compute temp also subtracts 2 degrees of freedom due to
translational invariance.

axel.

Thank you very much, Axel.

Best regards

Mario