From: "Michael Shirts" <[EMAIL PROTECTED]>
Reply-To: Discussion list for GROMACS users <gmx-users@gromacs.org>
To: gmx-users@gromacs.org
Subject: Re: [gmx-users] confused about rcoulomb<=rlist
Date: Wed, 12 Jul 2006 12:25:05 -0400
Hi, all-
I admit I've been wondering about this as well -- I was involved in
some of the previous discussions. It seems like the simplest thing is
to have the real space cutoff be at rcoulomb, and if one wants the
real space cutoff to go out to rlist, then one sets rcoulomb = rlist.
But one is not forced to. Am I missing something?
I think part of the previous discussion was how to get the best energy
conservation -- Berk argued (correctly, I think -- at least it appears
in agreement with simulations I ran) that setting the real space
cutoff = rlist gives the best energy conservation. But this is
independent of the question of user control -- I think the user should
be able to choose real space cutoff < rlist, for the reasons David
Mobley lists above.
So I guess that we agree that for production simulations there is
not much use for having rcouloumb != rlist with PME.
For studying effects of cut-off and integration setup on the results
I can see that one would to be able to set rcoulomb < rlist.
This is, however, not easy to implement exactly.
An exact cut-off would produce an infinit force at the cut-off.
But since we use spline interpolated tables, there will always
be some smoothing of this cut-off.
I think that with the current table setup there is no way to
have exact cut-offs.
Another option would be to apply a switch function to the PME
real space interactions.
Addionally one would want to set beta independent of rlist.
So the only reasonable option I can think of is to implement
a new coulombtype = PME-switch, but with a note that this
is mainly useful for testing purposes and the normal PME
will give more accurate results.
Berk.
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