[gmx-users] Modifying gromacs

Sai Pooja saipooja at gmail.com
Mon Nov 1 20:28:46 CET 2010

On Thu, Oct 28, 2010 at 10:57 PM, Mark Abraham <Mark.Abraham at anu.edu.au>wrote:

> On 29/10/2010 10:14 AM, Sai Pooja wrote:
>> Hi,
>> I want to modify the contribution to Interaction energy of different
>> groups - say I have groups A and B and I want the energy to be scaled as
>> E_AA + 0.1E_AB + 0.5*E_BB. Interaction parameters of each of these groups
>> are set by a forcefield
>> Now after multiple correspondence on the gromacs list I have concluded
>> that there are 3 ways of doing this:
>> 1. Using tables - for this I would have to list non-bonded parameters for
>> all atoms such that the combination rule and the table-potential is used.
>> For the table for BB interaction, scale the Coloumb and VdW interactions in
>> the tables by a factor of 0.5 and so on...
> Sure. I think that for the above example, you'd need only 2 (maybe 3) table
> files.
>  However, since tables would have to be supplied for pairs too (tablep), it
>> may not be accurate to supply 6-12 tables with coulomb potential for these
>> pairs. I am using CHARMM and the 1998 paper on CHARMM says that in some
>> specific cases the 1-4 interactions many be scaled which makes me doubt this
>> approach.
> Yeah, something extra will be required here. Obviously, test and develop on
> a small toy system that you can compute by hand in a spreadsheet.
>  2. Forcefield parameters - By defining scaled [nonbonded_params] for all
>> relevant atoms. This will change the VdW interactions, but not sure about
>> the Coulomb interactions.
> The Coulomb interactions are based on atomic charges, and there's no ready
> way to scale them differently for different interactions.
> I noticed that an option pairs_nb for LJ/C parameters is listed in table
5.4 in the Gromacs 4.5 manual. Can that be used to modify non-bonded
interactions-  both coulomb and LJ between different groups in forcefield
nonbonded files?


>  3. Modifying gromacs - by passing a parameter lambda to gromacs which
>> scales the force/potential by a factor lambda when gromacs calculates
>> force/potential.
>> For implementing option 3, which programs in the gromacs package would be
>> the bes tstarting points for editing the energy contributions of different
>> groups/atoms?
> I can think of two ways of approaching this. Efficiency requires that you
> use the energy group mechanism for your respective groups. Then you need to
> identify the generated non-bonded lists and do the necessary book-keeping to
> allocate scale factors to them. Then, either
> a) pass the scale factor all the way into the non-bonded kernels and use
> them suitably there, or
> b) create a copy of the force and energy arrays for each required scale
> factor, pass matching arrays into the appropriate kernels, and do the
> scaling on the respective arrays after all kernel contributions have been
> made, and then add the corresponding array elements.
> Either way, you'll need a sound understanding of the code in
> src/gmxlib/nonbonded.c (and the kernels it calls, and how its data
> structures get created in the neighbour-searching). Make a test system, like
> a dipeptide in a small box of water, with energy groups, and use a debugger
> to see how things work.
> a) is the easiest to develop. The kernels already have a "user data"
> pointer you can use, and it would be a fairly simple matter to adapt the
> generic C kernels to do your scaling. You will give up a lot of performance,
> however, unless you are prepared to adapt the hardware-optimized kernels
> similarly (not advised).
> b) will use somewhat more memory, have a smaller code-change footprint,
> keep essentially the same performance
>  As a more general question, how does one run a generalized Hamiltonian REM
>> on gromacs?
> You can't. GROMACS REMD requires a normal MD Hamiltonian supplied in a
> .tpr.
> Mark
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Quaerendo Invenietis-Seek and you shall discover.
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