[MMTK] Lennard Jones and Ewald Summation

Konrad Hinsen hinsen@cnrs-orleans.fr
03 Sep 2002 15:24:28 +0200


Berit Hinnemann <Berit.Hinnemann@fysik.dtu.dk> writes:

> -For a periodic universe, if I choose the default method, how many
> images are taken into account when calculation the Lennard-Jones energy.

One, in other words, the minimum-image convention is used.

> 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

There are changes up to a cutoff equal to the cell length. However, once
you go beyond half the cell size, the region with which any one particle
interacts is no longer spherical.

> up to 10AA (unit cell length is 7AA). So what is the internal cutoff
> radius?

If you see changes beyond the cell length, I'd like to see an example
script for verification. I don't see how this could happen.

> -In the Ewald summation, is it right that the internal parameters are
> set such that the relative energies are converged? I tried increasing

The default parameters are such that there is little risk for
non-convergence even for a rather ill-conditioned system. However,
this is based on a rule of thumb, so there is no guarantee. For
serious use, you should always determine parameters that are good
enough and optimal for your system.

Konrad.
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