[gmx-developers] Excluded charges and PME correction

[Berk Hess]

If you have exclusions in your topology, the ewald_LRcorrection will
completely remove
the mesh electrostatic forces, up to the mesh precision.
But a single dipole in a box will feel the effect of the periodic images.
Depending on the size of your box, this can be a large force.

Berk

David van der Spoel wrote:
> If I have a diatomic molecule with opposite charges, e.g. C=O, which
> are excluded in the short range, there will be a small force between
> the atoms due to PME. Is it correct that this force is completely
> taken away by the routine ewald_LRcorrection?
>
> For a simple test system (I2 oriented along the Y axis, with charges
> +/- 10 and bond length at 0.4 nm) we find at time step zero for bond
> completely in equilibrium:
>
> With cut-off:
>    f (2x3):
>       f[    0]={ 0.00000e+00, -2.24787e-03,  0.00000e+00}
>       f[    1]={ 0.00000e+00,  2.24787e-03,  0.00000e+00}
>
> With PME (epsilon_surface = 1)
>    f (2x3):
>       f[    0]={ 3.41797e+01, -1.11967e+02,  1.44752e+01}
>       f[    1]={ 3.41948e+01,  7.41194e+01,  1.44833e+01}
>
> With PME (epsilon_surface = 0)
>    f (2x3):
>       f[    0]={ 3.41796e+01, -1.73351e+03,  1.44758e+01}
>       f[    1]={ 3.41922e+01,  1.69567e+03,  1.44829e+01}
>
> Surely this can not be correct?
>
> This is using the git head sources.
>