I have a question on correctly choosing the parameters for the pppm method when running with boundary ppf and kspace_modify slab.
Currently, I use:
boundary p p f
kspace_style pppm/electrode 1.0e-6
kspace_modify slab 3.0
and let LAMMPS decide on gewald and the mesh. However, I am astonished about the choice that Lammps makes. What I get is the following:
G vector (1/distance) = 0.33944558
grid = 45 48 270
stencil order = 5
However, my system has a size of about 28 29 106 and consists of two metal slabs on top and bottom and water in between. Since I use kspace_modify slab 3.0, I would have expected a grid that is more or less proportional to 28 29 3*106 as the particle density (at least in the water, which makes up the largest part of the cell, is the same in x, y and z). I would therefore expect something like 45, 48, 512.
I looked into the code to try to figure out what Lammps actually does to set the parameters, but it did not really want to become clear to me what lammps does exactly.
So my question boils down to: Are the automatic estimates for the grid reliable when using kspace_modify slab, or is the estimated grid parameter in z artificially low, because the average particle density is artificially reduced in z?
Thank you for your help!