VMD-L Mailing List

From: Bart Bruininks (b.m.h.bruininks_at_rug.nl)
Date: Sat Jun 06 2020 - 14:47:30 CDT

 Thanks I was afraid of something like that, but thanks for pointing out
the cKDTree!

Op vr 5 jun. 2020 om 17:34 schreef Josh Vermaas <joshua.vermaas_at_gmail.com>:

> Hi Bart,
>
> I think you'd need to cycle over all the residues, and attack each residue
> one at a time. Since the loops are slow, and pbwithin needs to recalculate
> a data structure each time its called, what I've done recently is to
> use/abuse cKDTree implementations in scipy to ask the question of what is
> close to what in a periodic sense. A snippet would look something like this:
>
> from scipy.spatial import cKDTree
> #More setup to get R, which is a matrix of all atomic coordinates, and
> select specific indexes onidxs.
> tree = cKDTree(R[onidxs],boxsize=boxdims[i]+np.finfo(np.float32).eps)
>
> From the built tree (which you'd only need to do once per frame) you can
> ask questions like how many pairs of interactions fall within a cutoff
> (tree.query_pairs(cutoff)), or how many atoms are within a radius of a
> given point. See the scipy.spatial documentation for more:
> https://docs.scipy.org/doc/scipy/reference/spatial.html.
>
> -Josh
>
> On Fri, Jun 5, 2020 at 7:40 AM Bart Bruininks <b.m.h.bruininks_at_rug.nl>
> wrote:
>
>> Dear VMDers,
>>
>> I was trying to calculate contacts between molecules of the same species.
>> Usually the VMD selection procedure is very fast and accurate compared to
>> other options (GROMACS, MDAnalysis, etc). However, I can't seem to prevent
>> every molecule from returning True to the selection below:
>>
>> resname X and pbwithin 8 of resname X
>>
>> This makes sense, for X is a multiparticle molecule, therefore it is
>> always in range of itself. Is there something neat I can do to prevent this
>> from happening (preferably without affecting calculation time too much).
>>
>> I did find something related about the 'gofr' module from 2008, but I am
>> not sure how relevant it is. Back then such a selection was not possible, I
>> wonder if this is still the case.
>>
>> Cheers,
>>
>> Bart
>>
>