[gmx-developers] Re: Fairly detailed question regarding cell lists in Gromacs in general and nsgrid_core specifically

[Pedro Gonnet]

Hello again,

I had another long look at the code and at the older Gromacs papers and
realized that the main loop over charge groups starts on line 2058 of
ns.c and that the loops in lines 2135, 2151 and 2173 are for the
periodic images.

I still, however, have no idea what the second condition in lines
2232--2241 of ns.c mean:

        /* Check if all j's are out of range so we
         * can skip the whole cell.
         * Should save some time, especially with DD.
         */
        if (nrj == 0 ||
            (grida[cgj0] >= max_jcg &&
             (grida[cgj0] >= jcg1 || grida[cgj0+nrj-1] < jcg0)))
        {
            continue;
        }

Does anybody know what max_jcg, jcg1 and jcg0 are? Or does anybody know
where this is documented in detail?

Cheers, Pedro


On Tue, 2011-07-05 at 16:07 +0100, Pedro Gonnet wrote:
> Hi,
> 
> I'm trying to understand how Gromacs builds its neighbor lists and have
> been looking, more specifically, at the function nsgrid_core in ns.c.
> 
> If I understand the underlying data organization correctly, the grid
> (t_grid) contains an array of cells in which the indices of charge
> groups are stored. Pairs of such charge groups are identified and stored
> in the neighbor list (put_in_list).
> 
> What I don't really understand is how these pairs are identified.
> Usually one would loop over all cells, loop over each charge group
> therein, loop over all neighboring cells and store the charge groups
> therein which are within the cutoff distance. 
> 
> I assume that the first loop, over all cells, is somehow computed with
> the for-loops starting at lines 2135, 2151 and 2173 of ns.c. However, I
> don't really understand how this is done: What do these loops loop over
> exactly?
> 
> In any case, the coordinates of the particle in the outer loop seem to
> land in the variables XI, YI and ZI. The inner loop (for-loops starting
> in lines 2213, 2216 and 2221 of ns.c) then runs through the neighboring
> cells. If I understand correctly, cj is the id of the neighboring cell,
> nrj the number of charge groups in that cell and cgj0 the offset of the
> charge groups in the data.
> 
> What I don't really understand here are the lines 2232--2241:
> 
>         /* Check if all j's are out of range so we
>          * can skip the whole cell.
>          * Should save some time, especially with DD.
>          */
>         if (nrj == 0 ||
>             (grida[cgj0] >= max_jcg &&
>              (grida[cgj0] >= jcg1 || grida[cgj0+nrj-1] < jcg0)))
>         {
>             continue;
>         }
> 
> Apparently, some cells can be excluded, but what are the exact criteria?
> The test on nrj is somewhat obvious, but what is stored in grid->a?
> 
> There is probably no short answer to my questions, but if anybody could
> at least point me to any documentation or description of how the
> neighbors are collected in this routine, I would be extremely thankful!
> 
> Cheers, Pedro
> 
>