  We perform an extensive combinatorial search for optimal nanostructured
hydrogen storage materials among various metal-decorated polymers using
first-principles density-functional calculations. We take into account the
zero-point vibration as well as the pressure- and temperature-dependent
adsorption-desorption probability of hydrogen molecules. An optimal material we
identify is Ti-decorated cis-polyacetylene with reversibly usable gravimetric
and volumetric density of 7.6 weight percent and 63 kg/m^3 respectively near
ambient conditions. We also propose ``thermodynamically usable hydrogen
capacity" as a criterion for comparing different storage materials.
