hi
Why is the group/group Self-Interaction Energy Negative for Same-Charge Graphene Sheets in LAMMPS PPPM?
system details
Atoms per graphene sheet 416 carbon atoms
Sheet 1 (type 6, D_graphene) Charge = +0.002403846 e per atom
Sheet 2 (type 7, U_graphene) Charge = -0.002403846 e per atom
Total charge — Sheet 1 +1.000 e
Total charge — Sheet 2 -1.000 e
sigma (C-C) 3.214 Å
epsilon (C-C) 0.117110 kcal/mol
Box dimensions 32.079 × 34.193 × 35.539 Å
Boundary conditions p p f (periodic in x,y; fixed in z)
Kspace style PPPM with accuracy 1×10⁻⁴
Kspace slab correction kspace_modify slab 3.0
LJ cutoff 12.0 Å
Graphene sheets Frozen (fixed) throughout simulation
Solvent SPC/E water + Na+ + Cl- ions
Input File Setup
I calculate the interactions using the group/group compute command in a rerun script.
The relevant commands are:
pair_coeff * * lj/cut/coul/long 0 0 # LJ set to zero, only Coulomb active
group solute type 6 7 # both graphene sheets
group solvent subtract all solute # water + ions
compute 1 solute group/group solvent pair yes kspace yes # AB
compute 2 solute group/group solute pair yes kspace yes # AA
compute 3 solvent group/group solvent pair yes kspace yes # BB
The special_bonds setting used:
special_bonds lj 0.0 0.0 0.0 coul 0.0 0.0 0.0
The Question
When I separate the solute group into individual sheets and compute:
compute 1 solute1 group/group solute1 # Sheet 1 with itself = A11
compute 2 solute1 group/group solute2 # Sheet 1 with Sheet 2 = A12
compute 3 solute2 group/group solute2 # Sheet 2 with itself = A22
I get the following results:
Compute V alue (kcal/mol) Expected (naive) Actual Sign
A11 (sheet1 with sheet1) -3.271 Positive (same charge = repulsion) NEGATIVE
A22 (sheet2 with sheet2) -3.267 Positive (same charge = repulsion) NEGATIVE
A12 (sheet1 with sheet2) +63.44 Negative (opposite charge = attraction) POSITIVE
My specific questions are:
• Why is A11 negative even though Sheet 1 has all same-sign charges (+0.002404 e each) which should repel each other?
• Why is A22 also negative for the same reason?
• Why is A12 positive even though Sheet 1 (+1e) and Sheet 2 (-1e) are oppositely charged and should attract?
• Does the negative sign in A11 and A22 physically mean there is attraction between the same-charge atoms?
• What role does the PPPM Ewald self-energy correction play in these values?