[gmx-users] Twin range cut-off's: rlist and rvdw

[Jussi Lehtola]

On Wed, 2009-05-27 at 14:33 +0200, Yan Chai wrote:

> It seems that in the twin range cut-off method, rlist does not only
> play a role as a cut-off for neighbor searching, but also as a cut-off
> for short-range interactions. Do I understand correctly?

If you're using Coulombic cutoffs, then yes. If you're using PME, then
AFAIK the part r > rcut will be performed in Fourier space.

> Do people think the effect of these inward moving and outward moving
> particles is small and  the  error  for  calculation can be tolerated?

When particles move in and out of the cutoff range it induces static in
the properties. Ideally for monocomponent systems (such as metals) the
cutoff should be in an (absolute) minimum of the pair correlation
function, i.e. there aren't many particles in the region where the
cutoff is made. For molecular systems the decision of a cutoff is
harder, since some pair correlations stay quite constant and there is no
clear cut way to make the decision.

The farther you go the fewer static you get. Roughly speaking, the
number of particles at cutoff range grows as r_cut^2, but for
Lennard-Jones the interaction weakens as r_cut^-6, resulting in an error
proportional to r_cut^-4.
-- 
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Jussi Lehtola, FM, Tohtorikoulutettava
Fysiikan laitos, Helsingin Yliopisto
jussi.lehtola at helsinki.fi, p. 191 50632
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Mr. Jussi Lehtola, M. Sc., Doctoral Student
Department of Physics, University of Helsinki, Finland
jussi.lehtola at helsinki.fi
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