Coulomb PME convergence criteria no specified?

[mrshirts]

I don't think PME convergence criteria are specified (gaussaina smoothing, number of grid points, etc). Most codes different in what convergence criteria they use, so one could get relatively different answers without specifying this.

[jchodera]

I believe our general philosophy is to define the force field for the convergence limit for (Particle Mesh) Ewald-based methods, and allow some level of tolerance in implementations as long as it does not deviate too much from the intended energies.

However, we don't specify how much deviation is "too much" or what metrics of deviation are to be measured. I think this is an area where we could develop some useful tools in the future.

For now, we could place a limit on the energy deviation stddev for a benchmark set of systems, for example. Or, as you suggest, specify a PME cutoff tolerance.

[mattwthompson]

SMIRNOFF currently offers no guidance beyond

a method like particle mesh Ewald should be used

In the current implementation .setEwaldErrorTolerance(1.0e-4) is used in the OpenMM export. The reason for this choice is historical and I can't argue for it in favor of other values or possible combinations of settings. These settings are not part of the Amber or GROMACS exports as the Ewald settings are not specified in files provided. The case of LAMMPS is a little different but not the most important one to consider here.