[gmx-users] using SETTLE for constraints

[Berk Hess]

Things are a bit confusing here.

SETTLE versus other constraint algorithm is chosen at the topology level
by putting [ settle ] or [ constraints ] sections in your topology.
With of the other constraint algorithms (LINCS or SHAKE) is used
for the constraints in the [ constraints ] sections is set in the mdp file.

For future Gromacs versions I would like to have the [ settle ] sections
in the water topology replaced with [ constraints ] and let mdrun
decide if to use settle.
>From a user point of view, this will not change much though,
since settle is only useful for water and a user (nearly) never
writes a topology for water.

Berk

> Date: Mon, 7 Sep 2009 18:28:05 +1000
> From: Mark.Abraham at anu.edu.au
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] using SETTLE for constraints
> 
> Dmitri Dubov wrote:
> > Dear GMX'ers,
> > 
> > I am confused with some basic questions about using SETTLE constraining algorithm in gromacs. 
> > 
> > 1) This solution of SHAKE is used, particularly, for spc water. So spc.itp file contains the [ settles ] section, and there is no another [ constraints ] within it. But the parameter "constraint-algorithm" in /water/grompp.mdp (see the gromacs native tutorial for spc water simulation) is set to "Lincs". What is the REAL treatment of water constraints in this case?
> 
> GROMACS transparently uses the best constraint algorithm that you allow 
> it use on a given molecule. The only way you can *not* use constraints 
> on water with the standard water .itp files is to #define FLEXIBLE, as 
> you will see in those .itp files. Since settle is specified there it is 
> used.
> 
> The choice of LINCS or SHAKE pertains only to molecules for which there 
> is no other constraint option. You can verify this by doing a quick 
> simulation of pure water specifying either of the above and seeing that 
> the results are identical. IIRC the .log file makes a note of what 
> constraints are being used.
> 
> > 2) From manual the run parameter "constraint-algorithm" has only two (LINCS or SHAKE) alternatives. How can I specify the SETTLE using?
> 
> See above. Looking for SETTLE in the manual index would have helped :-)
> 
> Mark
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