Re: [ESPResSo-users] Negative Total Energy

[Olaf Lenz]

Anyway, what is the problem of getting negative energies? The Coulomb potential is negative for opposite charges, so when the particles start to form some structure, you will get negative energies.

It is more surprising that you do get 600kT in the first case. Apparently, the system behaves like a gas, and the particles do not interact. Did you wait for equilibration?

Olaf

2014-03-27 17:56 GMT+01:00 Olaf Lenz <address@hidden>:

What's the bjerrum length?

Olaf

2014-03-27 17:13 GMT+01:00 roya moghaddasi <address@hidden>:

Hi dear Olaf,
Thank you for your reply,
My box length is 150. I have a cylinder of radius 25 and length 150. My system are periodic in 3 directions. Within the cylinder, there are 1000 particles with charge +1 and 1000 particles with charge -1. The coulomb algorithm is P3M. There are 6-12 LJ interactions between particles with these parameters:
particle_sigma   1.0
particle_epsilon 1.0
particle_cutoff  1.12246
particle_shift   0.25
, and 3-9 LJ interactions between particles and cylinder constraint with these parameters:
constraint_sigma   1.0
constraint_epsilon 1.0
constraint_cutoff  0.858374
constraint_shift   0
In addition, I have thermostat langevin 1.0 1.0.
Are these informations enough?
Thank you again,
Roya

On Thursday, 27 March 2014, 20:31, Olaf Lenz <address@hidden> wrote:

The values on the system size, the temperature, the charges and the LJ parameters. If you do not give these, we cannot say whether it is reasonable.

Olaf

2014-03-27 16:51 GMT+01:00 roya moghaddasi <address@hidden>:

Hello everybody,

At first, I had 400 particles in my system, half of them where negatively charged, and half of them are positively charged. The potentials between the particles are LJ interaction and coulomb interaction. when I ran my program the total energy was almost 600KT which was reasonable in according to equipartition theorem.

But when I put 2000 particles in my system in the same circumstances, I got the -10^4KT for my total energy. It is negative and high. Is it reasonable. Why this happens?

I would appreciate if someone can help me.

best,

Roya

--

Dr. rer. nat. Olaf Lenz

Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart

Phone: +49-711-685-63607

--

Dr. rer. nat. Olaf Lenz

Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart

Phone: +49-711-685-63607

--

Dr. rer. nat. Olaf Lenz

Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart

Phone: +49-711-685-63607