Han Hu, you asked about obtaining per-atom stress/energy from the PPPM
"The electric field and forces are obtained by interpolating from grid to
atoms. Could the similar method be used to interpolate the potential and
virial stress from the grid back to the atoms?"
I have some beta code that does exact this--i.e. it obtains per-atom
stress and energy for the PPPM kspace method in LAMMPS. Perhaps you
could easily modify it for tip4p. I will send you a copy.