(3.4.9) Galli : As indicated earlier, interfaces between metallic and insulating phases and interfaces between low-gap and wide-band gap semiconductors remain challenging to describe with existing DFAs -namely, band offsets of these interfaces are not accurate with most functionals and even structural properties in some cases 367 turn out to be inaccurate. The electronic properties of several transition-metal oxides are equally challenging to describe with existing DFAs, especially those considered to be highly correlated materials. Empirical fixes have been proposed and used, as in the DFT+U method, but their predictive power is yet unclear, especially in cases where different values of U must be used for different oxidation states of the metal in the same oxide. An outstanding open problem is, for example, the metal-to-insulator transition in vanadium oxide (just to name one transition-metal oxide) as a function of oxygen composition or temperature.