[gmx-developers] calculating interaction energy for more than 256 groups

[David van der Spoel]

Florian Haberl wrote:
> Hi,
> 
> i posted this 3 days ago in gmx at users, and no answer, if there is no easy 
> solution this would be a nice improvement for upcoming gromacs versions:
> 
> i want to calculate an interaction energy matrix for more than 256 energy 
> groups.
> 
> First grompp only accept 255 groups so  i changed in readir.c 
> 
> #define MAXPTR 510
> #define NOGID  511
> 
> (from 254 and 255)
> 
> After that grompp writes out an tpr file and mdrun works without problems, but 
> than in the log file there are only interactions calculated for the first 256 
> energy groups.
> 
> what do i have to change also or is this somewhere an hard limit?
> 
> greetings,
> 
> Florian
the problem is that the group number is stored in a 8 bit variable, 
hence the 256. There is no easy solution, but if you need less than 512 
groups you could split the system in 2x256 groups, and do the simulation 
4 times to build up a matrix.

-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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