  The microscopic mechanism of the hydrogen-induced metal-insulator transition
in SmNiO$_3$ is clarified by means of density-functional theory with the
Hubbard U correction. While 100% of hydrogen doping per Ni atom has been
supposed to be responsible for the metal-insulator transition, we found that
50% of hydrogen doping results in an outstandingly stable atomic structure
showing the insulating property. The stable crystal structure shows the
peculiar layered pattern of charge disproportionation of Ni$^{2+}$ and
Ni$^{3+}$ valences together with the strong Jahn-Teller distortion that causes
the eg orbital state splitting and opens the band gap.
