# LAMMPS Tutorial 12- Energy not constant in NVE and Temperature different than what mentioned in NVT.

Dear All,

In the LAMMPS tutorial of studying the behavior of polymer chain (tutorial 12), nve has been used to equilibrate at 5000K. In the output file the total energy does not remain constant though nve ensemble is used. Also while minimization, nvt is used for 500K. But still in the output, the temperature is 4300K. Since, the temperature given is 500K in nvt, why the temperature is 4300K. And what is the need to equilibrate at such a high temperature of 5000K.

As this is the tutorial problem, this has to be correct but I don’t understand that why energy is not constant in nve ensemble. And what exactly is the use of fix langevin command in the script?

Dear All,

In the LAMMPS tutorial of studying the behavior of polymer chain (tutorial 12),

There is no “tutorial 12” in the regular lammps distribution. So it has to be somewhere else, or in contributed files and before going into any details, you have to provide more details what this refers to, and you will likely have to contact the authors of that specific tutorials to ask for corrections.

nve has been used to equilibrate at 5000K. In the output file the total energy does not remain constant though nve ensemble is used. Also while

Using fix nve does not guarantee conservation of the total energy. This is a misinterpretation of the name of the command and how it work. What fix nve does is to do time integration AND NOTHING ELSE. If your setup is otherwise correct and doe no other manipulations and has full periodic boundary conditions, this will result in an NVE ensemble. The same goes for fix nvt/npt BTW.

minimization, nvt is used for 500K. But

Time integration fixes have no effect during minimization and velocities remain unchanged.

still in the output, the temperature is 4300K. Since, the temperature given is 500K in nvt, why the temperature is 4300K. And what is the need to equilibrate at such a high temperature of 5000K.

I don’t know for certain, at (unphysically) high temperatures, you can have transitions that are not possible otherwise. Yet the results are easier to equilibrate into the desired state. If you start from a regular structure, you need to break up the regularity or else your system will be overly ordered and the results less meaningful.

As this is the tutorial problem, this has to be correct but I don’t understand that why energy is not constant in nve ensemble. And what exactly is the use of fix langevin command in the script?

It seems to me that your problems are mostly due to the lack if understanding in general concepts of molecular dynamics. How fix langevin interacts with a system is described in its documentation. My guess is that it acts as a “dissipating thermostat”, which is helpful to enforce equipartitioning across the system.

Axel

there is a tutorial 12 in the Society of Rheology tutorial posted

on the web site - but I don’t see any polymer example in it.

Steve