  We derive an exact formula of orbital susceptibility expressed in terms of
Bloch wave functions, starting from the exact one-line formula by Fukuyama in
terms of Green's functions. The obtained formula contains four contributions:
(1) Landau-Peierls susceptibility, (2) interband contribution, (3) Fermi
surface contribution, and (4) contribution from occupied states. Except for the
Landau-Peierls susceptibility, the other three contributions involve the
crystal-momentum derivatives of Bloch wave functions. Physical meaning of each
term is clarified. The present formula is simplified compared with those
obtained previously by Hebborn et al. Based on the formula, it is seen first of
all that diamagnetism from core electrons and Van Vleck susceptibility are the
only contributions in the atomic limit. The band effects are then studied in
terms of linear combination of atomic orbital treating overlap integrals
between atomic orbitals as a perturbation and the itinerant feature of Bloch
electrons in solids are clarified systematically for the first time.
