[gmx-developers] introduce effective potential

[Berk Hess]

>I would like to replace the non-bonded potential energy functions
>(Coulomb and vdW) by the effective potential
>
>U_eff = KT*q/(q-1)*log(1 + (q-1)*[U + epsilon]/KT)
>
>where q and epsilon are the parameters, U is the original
>nonbonded potential.
>
>This is equivalent to introduce the scale factor:
>
>q/(1 + (q-1)*[U + epsilon]/KT)
>
>into the forces which acting on the atoms.

Aside from practical problems with the code there are other problems.
For most bonded interactions it is possible.
But for non-bonded interactions you will run into conceptual problems.

Coulomb potentials can be both postive and negative, depending on
the signs of the two charges. Maybe you could circumvent this by
using abs(U) instead of U in your formula.
But Lennard-Jones interactions can be positive and negative
depending on the distance. One option could be splitting the
repulsion and the dispersion, but I don't know if this is what you want.

For the normal potentials one does not need to worry about how
the terms are split up, since the potentials are additive.
But your conversion is non-linear.

How things need to be done (or if it can be done at all) depends
on what exactly you want to use this effective potential for.

Berk.

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