Different velocities with 2 different commands!

Dear LAMMPS users and developers,

I am trying to compute the average velocity of a substance that is composed of 2 molecules. For this purpose, I am using 2 different methods:

1- Computing the kinetic energy of the substance with the command :

compute KinEn substance ke

Then, calculating the velocity with : v= sqrt(2E/m) , where m is the mass of the substance.

2- Computing the velocity directly with vcm/chunk command as follows:

compute cc1 substance chunk/atom molecule
compute myChunk substance vcm/chunk cc1

Then, I print the results with:

thermo_style custom step c_KinEN c_myChunk[1][1] c_myChunk[1][2] c_myChunk[1][3] c_myChunk[2][1] c_myChunk[2][2] c_myChunk[2][3]

However, the results that I obtain with the 2 methods are very different !

Therefore, I would like to ask you if there is something wrong with my code or if there is a concept in the vcm/chunk method that I am misunderstanding.

Thank you,

There is not enough information here to make any specific statements.

Please also note, that vcm computes the center of mass velocity.