  The concept of an optimal electron-phonon interaction spectral density as an
Einstein spectrum which allows to describe all physical properties of a
superconductor in an optimal way is developed from Carbotte's original
definition of an optimum spectrum. It is shown, using the borocarbides YNi2B2C
and LuNi2B2C as examples, that such a concept is meaningful even for
anisotropic systems. An Einstein spectrum is sufficient for clean-limit
systems, a 2$\delta$-peak spectrum is better suited for anisotropic systems
with impurities.
