  For two coupled Anderson impurities, two energy scales are present to
characterize the evolution from local moment state of the impurities to either
of the inter-impurity singlet or the Kondo singlet ground states. The high
energy scale is found to deviate from the single-ion Kondo temperature and
rather scales as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction when it
becomes dominant. We find that the scaling behavior and the associated physical
properties of this scale are consistent with those of a coherence scale defined
in heavy fermion systems.
