[ESPResSo-users] Weird results from P3M interaction?

[Clemens Jochum]

Dear all,

I get some weird (unphysical?) results from my simulation. Therefore I
have a couple of questions:


1) I have a system with one large charged dendrimer and counter-ions.
Additionally, there are charged salt ions. All particles interact via
LJ-interaction and Coulomb interaction (P3M). I attached some snapshots
of the resulting system at the end. In the snapshot the large dendrimer
is at the center of the particles. And (this is the confusing part) the
free ions seem to cluster with the clusters forming a regular lattice.
This does not seem right. My first guess is that the Coulomb interaction
and therefore the P3M method is the cause. Does anyone have an idea?

This is the output of the P3M tuning method:

P3M tune parameters: Accuracy goal = 1.00000e-04 Bjerrum Length =
7.00052e+00
System: box_l = 1.27568e+03 # charged part = 3326 Sum[q_i^2] = 3.32600e+03
mesh cao r_cut_iL     alpha_L      err          rs_err     ks_err    
time [ms]
16   3   1.79754e-01 1.07208e+01 9.97082e-05 7.071e-05 7.030e-05 3.63
16   2   2.46081e-01 7.66386e+00 1.19662e-04 7.071e-05 9.654e-05
accuracy not achieved
16   4   1.57645e-01 1.23319e+01 9.92441e-05 7.071e-05 6.964e-05 2.65
16   5   1.48033e-01 1.31872e+01 9.97046e-05 7.071e-05 7.029e-05 2.51
16   6   1.43227e-01 1.36592e+01 9.84446e-05 7.071e-05 6.849e-05 2.55
16   7   1.39382e-01 1.40609e+01 9.93436e-05 7.071e-05 6.978e-05 2.71
142  5   2.19736e-02 9.93374e+01 9.98180e-05 7.071e-05 7.045e-05 136.16
142  4   2.42866e-02 8.94035e+01 9.40771e-05 7.071e-05 6.205e-05 134.00
142  6   2.13954e-02 1.02165e+02 9.39748e-05 7.071e-05 6.190e-05 136.81
142  3   2.94909e-02 7.28644e+01 9.67037e-05 7.071e-05 6.597e-05 132.60
142  2   5.26210e-02 3.95352e+01 9.90264e-05 7.071e-05 6.933e-05 131.29
142  1   1.48033e-01 1.31872e+01 2.75994e-04 7.071e-05 2.668e-04
accuracy not achieved

resulting parameters:
16   16   16   5   1.48033e-01 1.31872e+01 9.97046e-05 2.51


2) When using the P3M method I sometimes get the warning "Statistics of
tuning samples is very bad" (But not for the simulation, which I used
for the snapshots). Is this a problem and on which parameters does this
depend on? Box size and accuracy?

3) The description of the "exclude" feature of particles says:"no
nonbonded interactions arecalculated". This means Coulomb interaction
for excluded particles are still calculated, correct?

4) I have several types of ions in my solvent, which I interact via the
same LJ-interaction, but I want them to have different LJ interactions
later on. So I want them as different types, which leads to 10 instead
of 3 LJ-interaction definitions. Is the computational cost for having 10
instead of 3 defined LJ-interactions very high? Keep in mind that the
actual number of interacting particles stays the same.

I'm thankful for any advice and input.

Best,
Clemens

System.png
Description: PNG image

System_Zoom.png
Description: PNG image