I am simulating an apparently simple system with two infinite charged and similar rods (as constraints) embedded in two cylinders (as constraints) neutralized by counterions in a 3d periodic box.
When I run this system with P3M and look at density maps for instance, they look totally fine and fit with my intuition of the physics going on in there.
However, when I try to compute the force on the rods (with the force constraint command), I always get an attractive result while it should be repulsive!
I have tried to change many things including accuracy of the P3M method, size of the box to prevent periodic images to interact with one another or rod charge density so that I get below the Manning condensation threshold and then I have naked charged rods interacting within a uniform background of opposite charges and I always get the same attractive force whatever I do.
Any idea on what could be going on?
Thank you very much for any kind of help you could provide.
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Fabien PaillussonDepartament de fisica fonamental