# Limitations in using FLEX as a DMFT solver

When using the fluctuating exchange approximation (FLEX) as a dynamical mean field theory (DMFT) solver, Kotliar, et al. (p. 898) suggest that it is only reliable for when the interaction strength, $U$, is less than half the bandwidth. How would one verify this? Also, is there a general technique for establishing this type of limit?

To clarify, DMFT is an approximation to the Anderson impurity model, and FLEX is a perturbative expansion in the interaction strength about the band, low interaction strength limit.

For perturbation theory in $U$, see the PRB 36, 675 (1986) by Horvatić et al. and references to and form that paper. A more comprehensive discussion can be found in the monograph by Hewson. As far as I remember, perturbation in $U$ on the level of self-energy does not give the expected exponential dependence on $U$ for the Kondo temperature.
• I've edited the answer to add a reference. You can see form the graphs that they choose moderate U's. For more details, see their references and the monograph by Hewson. As far as I remember, perturbation in $U$ on the level of self-energy does not give the exponential dependence on $U$ for the Kondo temperature – Slaviks Sep 15 '11 at 16:05