Marcus theory (Equation ) is another approach, however it is more tenuous to employ in the solid state as initial and final electronic states are localised, while in practice it is more likely one is delocalised. While this did not deter some , their results were less than stellar. Marcus theory can also be extended to phonon degrees of freedom as well as the electronelectron coupling. This can also be called quantum charge transfer theory , and while it does have a significantly different temperature relation compared to classical Marcus theory, phonon quantum tunnelling effects can be included in the physical description of the system. Marcus theory can be described by a Landau-Zener transition, but the Born-Oppenheimer approximation can break down for some systems, similar to how it fails at conical intersections in molecular systems; the initial and final potential energy surfaces are connected under the diabatic approximation, ie. conical intersection. To circumvent this issue, the wavefunction can be described as two uncrossed states. This allows a new coupling constant to be defined; known as static coupling theory . Alkauskas and coworkers studied gallium nitride semiconductors using the Shockley-Read-Hall model, but found underestimated results compared to experiment, arguing that static coupling would have worked better.