[gmx-developers] ewald shift

[Harold Park]

Hi Berk:

My apologies for posting on the same list.  Regarding the utility of  
the Wolf paper - it does not artificially truncate the Coulomb  
interactions at a cut-off.  It does use a spherical cut-off radius,  
but then it corrects the Coulomb energy by ensuring charge  
neutralization.  Doing so leads to a very simple expression for the  
Coulombic energies (equation 5.10) and forces (equation 5.21).  In  
that way, the Coulombic energy (and thus forces) can be calculated  
exactly using spherical truncation, as is often used in MD  
simulations, with a small cut-off radius.

I am aware that there are other methods that are used for non-periodic  
systems in slab geometries, but the Wolf method is general, and thus  
not utilize arguments to make the slab non-periodic in certain  
directions, while using Ewald in the other periodic directions.  In  
contrast, the Wolf method can be used both at surfaces and for bulk  
systems.

Regards,

Harold

> --
>
> Message: 1
> Date: Fri, 14 Aug 2009 16:36:05 +0200
> From: Berk Hess <hess at cbr.su.se>
> Subject: Re: [gmx-developers] ewald shift question
> To: Discussion list for GROMACS development
> 	<gmx-developers at gromacs.org>
> Message-ID: <4A857655.9090009 at cbr.su.se>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hi,
>
> Please do not post the same question on multiple lists.
> People might answer the same questions multiple times and the  
> discussion
> gets confusing.
>
> You can simply call shift_LRcorrection if you want, right?
> Also ewald_LRcorrection does nearly the same thing, but with a  
> different
> functional shape.
> Both function are called at nearly the same place in do_force_lowlevel
> in src/mdlib/force.c.
>
> But having looked quickly trough the paper, it would think that you
> don't need these functions
> at all.  You want to shift the pair potential for all pairs, or am I  
> wrong?
> Both functions in Gromacs only operate on excluded pairs.
> Shifting a pair potential can be done simply with user tables.
>
> BTW I don't see the advantage of this electrostatics method.
> Artificially truncating all Coulomb pair interactions at a cut-off is
> exactly the same
> as using a pure cut-off (although this method is probably
> computationally more efficient).
> I would consider the artifacts due to this worse than the artificial
> periodicity introduced by
> Ewald and PME.
> For interface systems Gromacs has a correction term for Ewald and PME
> (ewald_geometry=3dc)
> which works very well and there are also exact methods in the  
> literature
> for this geometry.