  Computing momentum-dependent susceptibilities in the dynamical mean-field
theory (DMFT) requires solving the Bethe-Salpeter equation, which demands large
computational cost. Exploiting the strong-coupling feature of local
fluctuations, we derive a simplified formula that can be solved at a
considerably lower cost. The validity and the physical meaning of the formula
are confirmed by deriving the effective intersite interactions in the
strong-coupling limit, such as the kinetic exchange and RKKY interactions.
Furthermore, numerical calculations for single-orbital and multiorbital models
demonstrate surprisingly wider applicability including weak-coupling region.
Based on this formula, we propose three levels of practical approximations that
can be chosen depending on complexity of problems. Simpler evaluations of spin
and orbital susceptibilities in multiorbital systems thus become possible
within DMFT.
