I have simulated a system including 1000 SPC/E water molecules with periodic boundary and pppm method. In the log file, I noticed that the stencil order of pppm equals 5:

PPPM initialization …

using 12-bit tables for long-range coulomb (src/kspace.cpp:342)

G vector (1/distance) = 0.19705771

grid = 50 50 50

stencil order = 5

I have read the source code of the pppm.cpp and find the stencil order is used in the calculation of the K-space error. In the paper provided by the manual, *Deserno and Holm, J Chem Phys, 109, 7694 (1998).*, the stencil order is represented by *P*, which is the number of species of charges. For SPC/E model, there are only two kinds of charges, -0.8472 and +0.4236, so *P* should equal 2, which contradicts the value in the log file.

Is there something wrong with my understanding? I have attached my code and result, and any help is appreciated!

stencil order problem.rar (387.3 KB)