# [gmx-developers] modifing nonbonded interactions

Berk Hess hessb at mpip-mainz.mpg.de
Fri Oct 24 09:44:41 CEST 2008

```Hi,

To understand the approximations, lets have a look at the simplest case:
two single-atomic ions in water.

In general the dielectric permittivity of the medium around a single ion is:
(1) eps(r)
When you have two ions this I already don't see a reasonable approximation
to describe the permittivity, but lets say for arguments sake that it is
still (1).
(If you read my papers you will see that even for high concentrations of
NaCl(aq)
the permittivity still seems near to constant of the whole space,
surprisingly this
even seems to hold for an Na-Cl contact pair!).

For two ions one can DEFINE the electrostatic interaction as:
(2) c(r)/r
Note that c(r) is NOT equal to 1/eps(r). To get c(r) from eps(r) you have
to solve the electrostatics problem with spatially varying permittivity.

For multi-atomic ions/molecules it is not clear how good approximation
(1) is.
For systems with more than two molecules (2) will be a (maybe bad)
approximation.
But using (2) in Gromacs with PME is possible, if c(r) is a constant
eps1 after the cut-off.
Then you should use:
coulomb_type = pme-user
epsilon_r = eps1
and fill the Coulomb table with eps1*c(r)/r.

But as Mark said, the real work will be in parameterizing the "LJ"
interactions,
which are no longer simply LJ, but contain all kinds of multi-body effects.

I think it would be much simpler and also not such a bad approximation
(especially compared to the many other approximations) to assume that the
permittivity is constant over space.

PS: Many people on this list seem to think that you do not need periodicity
with implicit solvent. This is only true when you have a single molecule
or a complex. In many cases, such as aggregation studies, one needs
periodic boundary conditions.

Berk

Mark Abraham wrote:
> Nazish Hoda wrote:
>>> PME requires periodicity, which you shouldn't need for implicit-water.
>> There are still charged molecules in the neighboring, next
>> neighboring, etc
>> boxes.
>> How to account for them ?
>
> If your system has a net charge, you have even more problems if you
> require no solvent (so no counter-ions) and periodicity. PME doesn't
> work for systems with net non-zero charge.
>
>>> If you're really wanting to do implicit-water simulation, you should
>>> be looking at simulation packages other than GROMACS.
>>
>> Won't using a modified LJ parameters work ?
>
> Life is not that simple. These parameters are carefully optimized to
> reproduce some (often vaguely) relevant data. It's inconceivable that
> any form of 6,12 potential on solute atoms could account for the
> electrostatic, frictional, thermostatic, dispersion and doubtless
> other effects that explicit solvents have upon their solutes. See
> http://wiki.gromacs.org/index.php/Parameterization
>
> Mark
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```