  Hydrogen exhibits qualitatively different charge states depending on the host
material, as nicely explained by the state-of-the-art impurity-state
calculation. Motivated by a recent experiment [Nature 546, 124 (2017)], we show
that the complex oxide SrCoO$_{2.5}$ represents an interesting example, in
which the interstitial H appears as a deep-level center according to the
commonly-used transition level calculation, but no bound electron can be found
around the impurity. Via a combination of charge difference analysis, density
of states projection and constraint magnetization calculation, it turns out
that the H-doped electron is spontaneously trapped by a nonunique Co ion and is
fully spin-polarized by the onsite Hund's rule coupling. Consequently, the
doped system remains insulating, whereas the antiferromagnetic exchange is
slightly perturbed locally.
