Dear LAMMPS users,
I’m a freshman using LAMMPS, so I apologize in advance for my probable simple problems.
When I use the pair_style “coul/long” and “coul/cut” to calculate the coulomb potential energy for a simple cubic lattice with net 0.5e electric charge for each point of the lattice, I find the coulomb potential energy with “coul/long” is negative, however that for “coul/cut” is positive. There are all 1000 atoms in the box.
Here’s some of my input for lattice and pair.
units metal #eV A ps
boundary p p p
lattice sc 10
region mybox block 0 10 0 10 0 10 units lattice
create_box 1 mybox
create_atoms 1 region mybox
mass 1 12
set group all charge 0.5
pair_style coul/long 11
pair_coeff * *
The coulomb potential energy for “buck/long” is -510.71783eV, however that for “coul/cut” is 1079.9734eV. The cutoff for coulomb potential is set to 11 angstrom.
Why is the negative coulomb potential energy from?