Re: Changing functions without recompiling

From: Axel Kohlmeyer (akohlmey_at_gmail.com)
Date: Mon Oct 18 2010 - 17:39:28 CDT

On Mon, Oct 18, 2010 at 5:53 PM, Cole Gleason <cole_at_colegleason.com> wrote:
> Hello all,

hello cole,

> I am a high school senior working with a few other students and my professor
> on some Molecular Dynamics research.  I'm handling the admin side of things
> by building and keeping our small cluster running.  What my professor wants
> to do is be able to change the functions and parameters of the pairwise
> interactions in simulations.  While I don't think that is too hard, the
> problem is that he wants to do this without recompiling the code every time.
>  He gave me some kind of Fortran90 function parser one of his friends wrote
> and wants me to allow the function parser to feed the equations to NAMD.

that is not a bad idea in principle.

>  The function parser seems to take a function string and create an internal
> representation of the function.  My professor suggested we use some kind of
> C library to call the fortran code.

there are better ways to do this.

> Now, I think this is the wrong approach.  It seems that even if this is
> possible, the performance hit will be significant.  Someone told me that I

exactly.

> should instead just rewrite the parts of the NAMD source code (or CHARMM
> perhaps?) we want to change and then somehow just recompile those sections.
>  Is that possible?  How do I go about doing that?

the best way to proceed would be to "tabulate" the functions.
i.e. produce a file that has the potential function and its resulting
force for each pair of particle types in a r, U(r), f(r) table.
this process would not be time critical and the tables can be created
easily even with a script language (python, perl, awk, tcl, ruby, and so on).
those tables would then be read in and used for computing
forces and energies during the MD simulation.

i know there was a hacked(?) version of NAMD that supports this kind
of "tabulation" floating around, but i don't know if this feature has made
it into the 2.7 release version.

but i know for sure that both the LAMMPS and the Gromacs MD codes
support this kind of feature for a long time now. i also have been thinking
to make that process easier by incorporating a function evaluator right
into the MD code, like your professor suggested, but have so far always
put this project on the back burner, since it is so easy to generate those
tables externally and just use them.

> Obviously this post is a little vague, so please tell me if you need any
> more information.  Thanks in advance to anyone who responds.

what would be interesting to know, is what functions exactly your
professor wants to look into. there are quite a few MD codes
available that support a _lot_ of different potential types/functions.
the parameters to those potentials can practically always be
changed without recompilation.

cheers,
    axel.

>
> --
> Cole Gleason
> ----------------------
> Student, Marmion Academy
> Email: cole_at_colegleason.com
> Website: colegleason.com
>

-- 
Dr. Axel Kohlmeyer    akohlmey_at_gmail.com
http://sites.google.com/site/akohlmey/
Institute for Computational Molecular Science
Temple University, Philadelphia PA, USA.

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