[gmx-users] limit on energygrp_table

[Berk Hess]

>From: "Adrien Leygue" <adrien.leygue at gmail.com>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: gmx-users at gromacs.org
>Subject: [gmx-users] limit on energygrp_table
>Date: Tue, 9 Oct 2007 18:13:21 +0300
>
>Dear Gromacs users (and developpers)
>
>I need to use a force field, derived from ab initio methods, for which
>the non-bonded interactions between the different atom types is a
>modified 3-parameters LJ potential.
>
>As this kind of potential is not implemented within Gromacs I am using
>fully tabulated user defined potentials V_i_j for the non bonded
>interactions between atom types i and j. The problem is that I have
>many different atomtypes (on the order of 20) and therefore I have to
>input many groups in the energygrp_table field of my mdp file.
>
>When I run grompp I have a message saying that there is a limit on the
>(254 if I remember correctly) on the number of groups that I can have
>in  the energygrp_table field.
>
>-Is there a way to increase this limit (by changing a constant and
>then recompile) ? or is it more complicated (from the value 254 one
>could guess that there is some one one-byte optimization)
>

This is indeed a on-byte optimization.
The group numbers are stored as uchars (include/types/atoms.h).
You could change this, then you also have to change 2 or more other files
for the io and communication of these numbers.

>-If the above solution is not an option, is there a "simple" way to
>modify the Gromacs source code to use these more advanced potentials
>while keeping good performances.

This depends on what combination rules you have for your parameters.
If you are lucky, you can use the buckingham 3-parameter setup
and modify the plain C buckingham innerloops to calculate your functional 
form.

Berk.

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