On Sat, Apr 7, 2012 at 2:57 PM, kirtana S <skirtana4.gmail.com> wrote:
> Sorry for not using the right terminology . I have used explicit solvent
> simulations.
> I want to calculate the change in solvation free energy of polymer in
> presence of ions.
>
> Is this only for implicit solvent models.
>
The MM/GBSA methodology is pretty much limited to implicit solvent models.
The problem with using explicit solvent models with these types of
calculations is that convergence is nearly impossible. For
moderately-sized periodic boxes, the total energy is dominated by
solvent-solvent interactions, which means that any protein-solvent or
protein-protein interactions will be drowned out completely for a handful
of snapshots. The only way you can get meaningful statistics is if you use
so many snapshots that the noise contributed by solvent-solvent
interactions is averaged out.
Keep in mind that noise will drop off as 1/sqrt(N) where N is the number of
frames in your averaging -- therefore, for large noise (which this is), it
will require a _large_ number of snapshots to get your desired interactions.
Implicit solvent, on the other hand, models equilibrium solvation
properties -- i.e., it should average over the solvent degrees of freedom,
completely eliminating the noise caused by explicit solvent.
Note that implicit solvent models can incorporate generalized ionic effects
via the introduction of a debye screening parameter (for GB) and the
Boltzmann part of the PB equation which can be truncated after the first
order term in the salt concentration (linear PB) or carried out to the sinh
term (non-linear PB). The manual for MMPBSA.py explains how to set salt
concentrations (or ionic strengths) for MM/GBSA and MM/PBSA.
Of course this will not capture the effect of structurally important ions
-- this is a much more difficult problem.
HTH,
Jason
--
Jason M. Swails
Quantum Theory Project,
University of Florida
Ph.D. Candidate
352-392-4032
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Received on Sat Apr 07 2012 - 12:30:03 PDT
