Dear All,

I am trying to understand the theory used to calculate temperature in Lammps. Statistical mechanics relates ‘the mean kinetic energy of the molecules or atoms to the temperature of the system’.

My question is that in Lammps how to consider the velocity (or kinetic energy) of the molecules or atoms within the simulation box if we move the whole box with some translational or rotational accelerations. what velocities should be used to calculate the kinetic energy and temperature: the velocity relative to the box or the velocities relative to some fixed reference frame? since these two are not the same any more.

The system’s temperature is not supposed to change due to only the whole box 's motion. ?

Thank you for your time and help!

Lili Zhang

Dear All,

I am trying to understand the theory used to calculate temperature in

Lammps. Statistical mechanics relates 'the mean kinetic energy of the

molecules or atoms to the temperature of the system'.

My question is that in Lammps how to consider the velocity (or kinetic

energy) of the molecules or atoms within the simulation box if we move the

whole box with some translational or rotational accelerations. what

velocities should be used to calculate the kinetic energy and temperature:

the velocity relative to the box or the velocities relative to some fixed

reference frame? since these two are not the same any more.

The system's temperature is not supposed to change due to only the whole box

's motion. ?

the total kinetic energy in lammps is the collected kinetic energy of all atoms.

in lammps it is assumed that the box in itself doesn't move or rotate. you can

have objects inside your system that do, but then the computed "temperature"

is still based on the total kinetic energy of its atoms. any setup

that would not

conform to this, would be in error. there are several discussions in the mailing

list archives on the "flying icecube syndrom".

axel.

Many of the temperature computes, e.g. compute temp.com,

perform their calculation of T after removing a "bias" from

the atom velocities, e.g. the velocity of the center of mass.

Thus various kinds of collective motion can be subtracted

from the atoms, before the thermal KE is used to calc a T.

See section 6.16 of the manual for a discussion.

Steve