Despite its relevance, the microscopic origin of the energy barrier, B, between the compressed and elongated geometries of Jahn−Teller (JT) systems is not well understood yet because of a lack of quantitative data about its various contributions. Seeking to clear up this matter, we have carried out both periodic and cluster ab initio calculations on the model system NaCl:Ni + . This system is particularly puzzling because, according to experimental data, its barrier is much smaller than that for other d 9 and d 7 ions in similar lattices. All calculations performed on the model system lead, in fact, to values |B| ≤ 160 cm −1 , which are certainly smaller than B = 500 cm −1 derived for NaCl:M 2+ (M = Ag, Rh) or B = 1024 cm −1 obtained for KCl:Ag 2+ . As a salient feature, analysis of calculations carried out as a function of the Q θ (∼3z 2 − r 2 ) coordinate unveils the microscopic origin of the barrier. It is quantitatively proven that the elongated geometry observed for NaCl:Ni + is due to the 3d−4s vibronic admixture, which is slightly larger than the anharmonicity in the e g JT mode that favors a compressed geometry. The existence of these two competing mechanisms explains the low value of B for the model system, contrary to cases where the complex formed by d 9 or d 7 ions is elastically decoupled from the host lattice. Although the magnitude of B for NaCl:Ni + is particularly small, the tunneling splitting, 3Γ, is estimated to be below 9 cm −1 , thus explaining why the coherence is easily destroyed by random strains and thus a static JT effect is observed experimentally. As a main conclusion, the barrier in JT systems cannot be understood neglecting the tiny changes of the electronic density involved in small distortions. The present calculations reasonably explain the experimental g tensor of NaCl:Ni + , pointing out that the d−d transitions in NiCl 6 5− are much smaller than those for CuCl 6 4− and the optical electronegativity of Ni + is only around 1.