[gmx-developers] PME-Switch vs. PME

[Michel Cuendet]

Dear Berk,

Thanks for your immediate reply. And I apologize for not providing the relevant cutoffs. Indeed I had a long switching range. 

OK Whow. So it is exactly as I feared, all my simulations are flawed. Being a big proponent of energy conservation, when 4.5 came out, I thought PME-switch was like the new state-of-the-art scheme (without properly checking what it was doing, my bad...). Now I realize that it is not. It is even quite wrong, as it probably introduces a sign reversal in the force around rcoulomb. As you mentioned, switching can be attained in a safe and consistent way using ewald_rtol. 

It seems to me that the PME-switch option is very dangerous. As you suggest I think it should be deprecated or at least it should be accompanied with a serious warning. 

Thanks for your help, 
Michel


On Mar 6, 2013, at 7:34 PM, gmx-developers-request at gromacs.org wrote:
> Message: 4
> Date: Wed, 06 Mar 2013 19:29:19 +0100
> From: Berk Hess <hess at kth.se>
> Subject: Re: [gmx-developers] PME-Switch vs. PME
> To: Discussion list for GROMACS development
> 	<gmx-developers at gromacs.org>
> Message-ID: <51378AFF.9020201 at kth.se>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> Hi,
> 
> You don't mention your, critical, cut-off parameters.
> I guess you used a long switching range. The switch is only intended to 
> avoid
> cut-off artifacts (and therefore does not act on the reciprocal).
> 
> Any switching function should only be applied over a very short range.
> As with PME ewald_rtol is usually 10^-5, you don't really need the switch,
> and if you do want to use it, you should switch over 0.05 nm or less.
> 
> But in 4.6 you can use exact cut-off's, either with the Verlet cut-off 
> scheme,
> or at a high cost also with the group cut-off scheme. Thus PME-switch is 
> no longer needed
> and we should deprecate it in 4.6.
> 
> Cheers,
> 
> Berk

==========================================================
Michel Cuendet, PhD
Molecular Modeling Group
Swiss Institute of Bioinformatics
CH-1015 Lausanne
Switzerland
http://lausanne.isb-sib.ch/~mcuendet/
==========================================================







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