[gmx-users] Modifying gromacs

[Sai Pooja]

I have a follow-up question:
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.
>
>
>  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
>


> Which of the two options will be more efficient -
>
> 1. Using tables for different energy groups (as in the case of option 1)
> and then writing a separate script that runs Replica exchange (i.e. not
> using Gromacs replica exchange). This would mean that the script will check
> Epotential every 1-4ps as the case may be.
>
> 2. Modifying Gromacs such that a user defined quantity(say lambda_user)
> scales the potential energy surface and the normal T based replica exchange
> can be used such that all replicas have a scaled potential energy function
> and are also at different temperatures.
>

Pooja

>
>  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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