[gmx-users] confused about rcoulomb<=rlist

[Michael Shirts]

> 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.

Perhaps I'm a little confused here.  Right now, isn't there an abrupt
cutoff at rlist anyway?  So the abrupt cutoff is just being moved
inward?  I suppose there's some technicality of the splines used to
generate the real space part, however. . .

> 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.

Right, one should be able to tune this independent of your
neighborlist cutoff for various reasons that David discussed.

> 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.

Actually, I can imagine situations where extremely good energy
conservation (for multi ns runs) is important, and this option might
be useful -- you would trade a bit of accuracy for better energy
conservation.

Best,
Michael