  We present a systematic study of the low-energy structures of zinc
borohydride, a crystalline material proposed for the hydrogen storage purpose.
In addition to the previously proposed structures, many new low-energy
structures of zinc borohydride are found by utilizing the minima-hopping
method. We identify a new dynamically stable structure which belongs to the
$I4_122$ space group as the most stable phase of zinc borohydride at low
temperatures. A low transition barrier between $I4_122$ and $P1$, the two
lowest-lying phases of zinc borohydride is predicted, implying that a
coexistence of low-lying phases of zinc borohydride is possible at ambient
conditions. An analysis based on the simulated X-ray diffraction pattern
reveals that the $I4_122$ structure exhibits the same major features as the
experimentally synthesized zinc borohydride samples.
