# [MMTK] Lennard Jones and Ewald Summation

**Berit Hinnemann
**
Berit.Hinnemann@fysik.dtu.dk

*Tue, 03 Sep 2002 14:15:19 +0200*

Hi,
I have a couple of questions to how the Lennard-Jones options and the
Ewald summation work:
-For a periodic universe, if I choose the default method, how many
images are taken into account when calculation the Lennard-Jones energy.
Is there some internal cutoff-radius or something? I have tried to
specify and to vary the cutoff-radius. If only the nearest image was
taken into account, there should be no changes in energy, when I set the
cutoff larger than half the unitcell length. However, there were changes
up to 10AA (unit cell length is 7AA). So what is the internal cutoff
radius?
-In the Ewald summation, is it right that the internal parameters are
set such that the relative energies are converged? I tried increasing
real/decreasing reciprocal the cutoffs. The absolute energies still
changed, until they converged at some point, but the relative energies
(energy difference between two different conformations) stayed the same
all the way.
Thanks,
Berit
--
Berit Hinnemann
Center for Atomic-Scale Materials Physics (CAMP)
Dept. of Physics, Technical University of Denmark
Building 307, DK-2800 Lyngby, Denmark
Phone: +45 4525 3209, Fax: +45 4593 2399