The Cp2k program is optimized for periodic calculations, and those are the default, still it is possible to perform also other kinds of calculations. First an important thing to note is that by convention the cell goes from 0 to h, not from -h/2 to h/2, thus if you want to put a cluster at the center of the cell you need to put it in h/2 not in 0.
The periodicity of the neighbor list, and thus of the gaussian collocation/integration is controlled by
The poisson solver (electrostatics) is controlled in the section
FORCE_EVAL%DFT%POISSON. The possible poisson solver are chosen with the
POISSON_SOLVER keyword, that can have the values
PERIODIC, which can do only fully periodic solvers,
ANALYTICwhich can do 0, 1d and 2d periodic solutions using analytical green functions in the g space (slow convergence),
MTMartyna Tuckermann decoupling, that interacts only with the nearest neighbor, beware these are completely wrong in the cell is smaller than twice the cluster size (with electronic density),
MULTIPOLE, uses a schema that fits the total charge with one gaussian per atom
The periodicity of the poisson solver (electrostatics) is set in
FORCE_EVAL%DFT%POISSON%PERIODIC, and the poisson solver chosen should support it.