fix electrode/conp: charge alternates sign between layers of a multilayer graphene electrode. Is it expected?

LAMMPS LAMMPS General Discussion
1

Hi all,

In a CPM simulation with 7-layer graphene electrodes, the sheet facing the electrolyte carries more charge than the electrode’s net total, and the sheet directly behind it carries charge of the opposite sign. Is this expected behavior of fix electrode/conp, or does it indicate a problem with my setup?

My system has two 7-layer graphene electrodes, separated by ~57 Å of 0.5 M aqueous NaCl. boundary p p f, slab-corrected PPPM, NVT at 323 K, 1 fs timestep. The outermost sheet of each electrode is held immobile.

I measured the sum of Qleft and Qright amounts to 0 to within 1e-15 and also the layer charges sum to the electrode.

My questions:

  1. Is alternating-sign charge across the layers of a multilayer electrode expected from fix electrode/conp, or does it point to something wrong in my setup?

  2. Is the effect controlled by eta? I am using 1.805132 Å⁻¹; the CONP2 README suggests 1.979 for graphene. Is there guidance for choosing it with a layered electrode?

My LAMMPS version: 11 Feb 2026, ELECTRODE package with pppm/electrode.

Much thanks in advance.

2

It is normal to see the charge “ringing”: see results from the original 2022 paper: https://pubs.aip.org/aip/jcp/article-abstract/156/18/184101/2841096/Fully-periodic-computationally-efficient-constant?redirectedFrom=fulltext

The underlying cause is that ELECTRODE solves for the charges needed to equalize electrostatic potential throughout the electrode. The resulting charge distribution would be monotonic if we treated the electrode as a continuum (roughly related to the skin depth), but at atomistic scale the particles are both discrete and laid out in ordered layers which leads to the effect you observed.

It is nothing to worry about as long as you clearly describe your simulation observables and you are not trying to be too fancy about what the individual layer charges mean. If you needed a completely accurate description of the charges within a conductor, you would need a quantum chemistry approach to describe those electrons anyway.