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

[Berk Hess]

Hi,

I don't think it matters much if you put the cut-off at a minimum in the rdf.
The real issue is that the force is not updated beyond the cut-off,
while the atoms there still move. You want as little motion,
of better as little change in force due to the motion of the atoms
as possible. The solution to this is to increase the cut-off as much
as possible, but this is computationally unfavourable.

I should also note the all versions of Gromacs up till now use
a very simplistic twin-cut off setup with an integrator which
is not reversible. This can lead to significant energy increase in the system.
Therefore twin-range cut-off should not be used for Coulomb interactions,
only for Lennard-Jones, where the forces are anyhow very small.
We might consider implementing a proper multiple time step integrator for
version 4.1 or 4.2.

Berk

> Subject: Re: [gmx-users] Twin range cut-off's: rlist and rvdw
> From: jussi.lehtola at helsinki.fi
> To: gmx-users at gromacs.org
> Date: Wed, 27 May 2009 16:05:25 +0300
> 
> 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.
> -- 
> ------------------------------------------------------
> Jussi Lehtola, FM, Tohtorikoulutettava
> Fysiikan laitos, Helsingin Yliopisto
> jussi.lehtola at helsinki.fi, p. 191 50632
> ------------------------------------------------------
> Mr. Jussi Lehtola, M. Sc., Doctoral Student
> Department of Physics, University of Helsinki, Finland
> jussi.lehtola at helsinki.fi
> ------------------------------------------------------
> 
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