That is not to say that the legacy models are, or were not, brilliant in their time. However, we simply understand and can calculate more using modern quantum chemistry techniques. Both Lin and Longeut-Higgins concluded that the current state of the art of internal conversion, based on the transition probability, fails whenever the adiabatic approximation is not applicable, or does not correctly describe the stationary state of the system. Sharf and Silbey noted that due to the strong dependence of the energy denominator on nuclear coordinates, the predicted coupling could be erroneous, smaller by ∼1 order of magnitude than compared to a result calculated using simple adiabatic functions as a zeroth-order basis set. The "crudeness" of the basis set is due to approximation employed where electrons are unable to "adiabatically" follow nuclear vibrations. In other words, the potential energy of the electrons is taken as a set of Coulomb attractions/repulsions (see . This results in negative charge centers, and therefore poor vibrational constants , at least compared to those obtained using an adiabatic basis set. However, the adiabatic basis should be appropriate for most cases; Nitzan & Jortner argue that as long as the basis set is complete with respect to the Hamiltonian and perturbative operators, then the issue is moot.