[gmx-users] PME accuracy

[Vincent Ballenegger]

Bert de Groot wrote:

>Somewhat worried by this statement (and the rest of the email), I performed 
>two reruns (with pme_order 4 and 6) to calculate the electrostatic part
>of the force on an ion in 200 MD snapshots (system: protein/lipids/water, 
>approximately cubic box with ~80.000 particles). The differences for each component 
>(and hence the sum) were very small: maximally 2 kJ/mol.nm (whereas the forces 
>themselves have typical values of +/- 1000 kJ/mol.nm. 
>
>So I don't know if the problems described by Vincent are particular to
>his system (unshielded charges as far as I could see), but at least I was 
>not able to reproduce the problem for a typical simulation system. 
>(at least not to the extent described). 
>
The inaccuracy in the forces appear mainly at large distances, where the 
forces are small (less than 70 kJ/mol.nm). I think that to see the 
problem in your system, you would need to calculate the rms error in the 
electrostatic forces specifically for the particles that are, say, more 
than 1.4nm apart.

Vincent

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