Berk Hess wrote:
Hi Berk,
Thanks for your feedback.
I think there is still a general issue with optimizing the PME settings
(or settings for any electrostatics method) for MD simulations.
The question is what exactly one should optimize.
If you do a single point calculation, you might only want to minimize
the force error. But for MD you might also want good energy conservation.
Optimizing the RMS force is certainly not the best choice,
since that would lead to a large ewald_rtol, which gives small
errors before the cut-off and on most of the long-range part,
but large errors just beyond the cut-off. This will lead to bad
energy conservation. Even an absolute force error will probably
lead to a too large ewald_rtol.
I think the optimization target function should be the RMS error over
the 3 components of the total force on each of the N atoms. Thus the RMS
is over 3N values. I can't see how this optimization process would seek
an (rcoulomb, ewald_rtol) point where large errors at r = rcoulomb+delta
exist, since this large error would propagate immediately to the error
in the total force on each atom, and persist through the subsequent RMS
averaging, since all atoms are affected similarly. If the RMS averaging
was over all contributions to the components of the total force on each
atom (which is not well-formed for Ewald anyway) then I could maybe see
such a problem arising.
I do agree that such an optimization should check for suitable energy
conservation also. Someone planning NVE has different requirements from
NVT, etc, and so they may need to optimize differently.
Mark
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