The structure and properties of a 16-atom body-centered cubic lithium cell with an interstitial hydrogen atom are studied using a pseudopotential-plane-wave method within the density functional theory at 0 K and high pressures.The host lattice is dramatically distorted by the introduction of H.Although the hydrogen atom is stable at the tetragonal site in perfect bcc host lattice,it favors the octahedral site formed by six nonequivalent Li atoms after full relaxation of the cell,showing P4/mmm symmetry within the pressures ranging from 0 to 6 GPa.The lattice ratio (a/c) changes irregularly with external pressure at about 3 GPa.The hydrogen band lies in the bottom of the valence band,separated by a gap from the metallic bands,illustrating the electronegativity of hydrogen.High reflectivity in the low frequency area induced by the impurity hydrogen is observed when only interband transitions are taken account of.A dip in reflectivity due to parallel band transitions is observed at ～0.4 eV.Another dip at ～4.3 eV appears when external pressure increases over 4 GPa.