The evaluation of vibronic couplings also requires correct description of at least two electronic states in regions where they are strongly coupled. This requires the use of multi-reference methods such as MCSCF and MRCI, which are computationally demanding and delicate quantum-chemical methods. This is further complicated by the fact that definition of vibronic couplings requires electronic wave functions. Unfortunately, wave function based methods are usually too expensive for larger systems and popular methods for larger systems such as density functional theory and molecular mechanics cannot generate wave function information.