Dear Prof. Stan,
I want to simulate the process of heterogeneous nucleation (containing ions (40 na+)) of supersaturated water vapor, and PPPM is
used to compute the long-range coulombic interactions. Thus, net charge of my system is not zero, there is a warning in lammps:
system is not charge neutral, net charge=40.
When the net charge of my simulation system is not zero, the ewald(PPPM) algorithms will act as if the simulation cell is surrounded
by an evenly distributed countercharge and thus neutralized. Besides, I find that a correction to the system energy is executed by
LAMMPS (In page 327 of the LAMMPS manual (compute group/group command), it says that “However, one can assume the
net charge of the system is neutralized by a uniform background plasma, and a correction to the system energy can be applied to
reduce artifacts. This correction term does not affect the force calculation and will be zero if one or both of the groups are charge
neutral. This energy correction term is the same as that included in the regular Ewald and PPPM routines.”
I have used the compute group/group command to check whether the correction term is added. I found that the energy is different,
but the force is the same, if the boundary keyword is set to yes and no.
So I want to know whether this energy correction is included into the PPPM method during the the calculation of particles’s
movement or it just work when the compute group/group command is used to calculate the energy (whether the
long-rang contribution to the force acting on the particle is ill-defined?). What is the evidence that I can use to convince
others?
Best regards
Zhang Chao