# [lammps-users] Temperature

Steve,

To calculate temperature of a group of particles, (Sum of KE=1.5NKbT) is used where N is the no of particles. In lammps, compute_temp is used for calculating temp of a group of predefined particles. In the source code, i observed that KE of atoms is divided by dof & Kb. I am not sure how dof is being calculated. If u look at the following lines,
the first step gives dof =3
N for a 3D strcture.
dof = domain->dimension * natoms;
dof -= extra_dof + fix_dof;

But i didn’t understand what the second line does. In my simulation using s s s boundary conditions, if i calculate the temp of a group of particles, final value of dof used is 3*(N-1) instead of 3N. I believe this is being changed in the second line. Can you please explain what exactly they mean? Although it doesn’t make much difference when u have a large value for N, but i am dealing with a small no of particles, which makes it significant.

Thanks
Ajit

Compute_modify extra sets the extra_dof value which is indeed 3 by default.
It is the dof for the motion of the center-of-mass of the group of
particles, and it's generally correct to subtract it. It's an open issue
for small #'s of particles, as is the concept of temperature.

The fix_dof value is contributed by other fixes which may remove
degrees of freedom, e.g. shake.

Steve

Hi Steve and LAMMPS users,

I had a specific question about the pair style hybrid. I am quoting the text wherein "The /hybrid/ style could be used for a simulation of a metal droplet on a LJ surface. The metal atoms interact with each other via an /eam/ potential, the surface atoms interact with each other via a /lj/cut/ potential, and the metal/surface interaction is also computed via a /lj/cut/ potential."

Now in this scenario, If eam has been used for the bulk metal and the lj/cut potential is used for the metal/surface interaction. How is the many-body eam potential matched with the lj/cut potential. Is there a danger of creating an artificial interface along the transition between the two potentials?

Thanks,
CD

How is the many-body
eam potential matched with the lj/cut potential. Is there a danger of
creating an artificial interface along the transition between the two
potentials?

That's up to you. Yes, there is a danger if you don't match the
potentials appropriately. You get to (mis-)specify the params of both
potentials.

Steve

Hi Steve,

Thank you for your answer. My next question. After going through the manual i could not find any clear indications as to how potential matching is done for the hybrid pair style. Is there any available resource which explains potential matching well or my best bet is to scour the available literature.

Thanks,
CD

Steve Plimpton wrote:

Again, it's up to you to match up the multiple potentials. LAMMPS does
not do this for you. But you have complete control over the params
of the individual potentials, so you can do whatever you wish.

Steve