Imposing periodic boundary conditions only on specific particles

Hello all,

I am trying to simulate a system where I want only a single polymer in a bulk medium. Imposing periodic boundary conditions ensures that solvent particles are not lost in the simulation. However I don’t want the polymer to be affected by its mirror image so I want the polymer to be able to exit the box but not the solvent. Is there a way to
a)ensure that only solvent particles follow periodic conditions and not the polymer.
b)ensure that the polymer or the solvents do not interact with the mirror image of the polymer(this gets broken if one end of the polymer crosses the wall, reappearing on the other side and interacts with itself on the other end

Your suggestions are greatly appreciated! Thanks!

No. But you can apply a position restraint to keep it from diffusing around.
You could also just let the polymer move freely and recenter the trajectory (either after the fact or using fix recenter. Note the difference between fix spring and fix recenter by carefully reviewing the respective documentation).

No. The only way to ensure this would be to increase the box size so that the interactions between the periodic images are small. How large or small those are depend on the properties and force field settings for both the polymer and the solvent.

Thanks for your clarification!

(optional question)
Is this considered as a useful feature from the simulation perspective which could be implemented in the future?

No. Implementing inconsistent behavior is never desirable. By treating atoms as if they don’t wrap around with periodic boundaries, you would leave “holes” in the principal cell and thus negate the very reason why you want to use periodic boundaries.