  The calcium intercalated graphite (CaC$_{6}$) is considered to be a
representative material of the graphite intercalated superconductors, which
exhibits sizable anisotropy of the Fermi surface (FS). Herein, the influence of
the FS anisotropy on the critical magnetic field ($H_{C}$) in CaC$_{6}$
superconductor is comprehensively analyzed within the Migdal-Eliashberg (M-E)
formalism. To precisely account for the mentioned anisotropy effects, the
analysis is conducted in the framework of the six-band approximation, hitherto
not employed for calculations of the $H_{C}$ function in CaC$_{6}$ material.
For convenience, the obtained results are compared with the available one- and
three-band estimates reported by using the M-E theory. A notable signatures of
the increased number of bands are observed for the temperature dependent
$H_{C}$ functions. In particular, the $H_{C}$ function decreases at T=0 K as
the number of the considered bands is higher. Moreover, it is argued that the
six-band formalism yields the most physically relevant shape of the $H_{C}$
function among all considered approximations. Therefore, the six-band model
introduces not only quantitative but also qualitative changes to the results,
in comparison to the other discussed FS approximations. This observation
supports postulate that the six-band model constitutes minimal structure for FS
of CaC$_{6}$, but also magnify importance of anisotropy effects for the
critical magnetic field calculations.
