  We have used calculations based on density functional theory to investigate
the energetics of hydrogen absorption in calcium-intercalated graphites. We
focus particularly on the absorption energy and the stability of the
hydrogenated material with respect to decomposition into graphite and calcium
hydride, which is essential if this material is to be useful for practical H2
storage. The calculations are performed with two commonly used approximations
for the exchange-correlation energies. Our calculations confirm earlier
predictions that the absorption energy is approximately -0.2 to -0.4 eV, which
is favourable for practical use of Ca-intercalated graphite as a hydrogen
storage medium. However, we find that the hydrogenated material is strongly
unstable against decomposition. Our results therefore explain recent
experiments which show that H2 does not remain stable in CaC6 but instead forms
a hydride plus graphite.
