  The discovery of superconductivity in the 122 iron selenide materials above
30 K necessitates an understanding of the underlying magnetic interactions. We
present a combined experimental and theoretical investigation of magnetic and
semiconducting Ce$_{2}$O$_{2}$FeSe$_{2}$ composed of chains of edge-linked iron
selenide tetrahedra. The combined neutron diffraction and inelastic scattering
study and density functional calculations confirm the ferromagnetic nature of
nearest-neighbour Fe -- Se -- Fe interactions in the ZrCuSiAs-related iron
oxyselenide Ce$_{2}$O$_{2}$FeSe$_{2}$. Inelastic measurements provide an
estimate of the strength of nearest-neighbor Fe -- Fe and Fe -- Ce
interactions. These are consistent with density functional theory calculations,
which reveal that correlations in the Fe--Se sheets of
Ce$_{2}$O$_{2}$FeSe$_{2}$ are weak. The Fe on-site repulsion $U_{Fe}$ is
comparable to that reported for oxyarsenides and K$_{1-x}$Fe$_{2-y}$Se$_{2}$,
which are parents to iron-based superconductors.
