Despite spin Hamiltonian parameters measured at T = 77 K for elongated centres in Ag2+ doped KCl and NaCl are nearly identical the transition temperature, T-t, for observing an isotropic EPR spectrum is however quite different. This puzzling situation is solved through CASPT2 calculations which lead to the same value of the equatorial Ag2+-Cl-distance in both systems but to a 15 pm difference between axial distances. This fact implies a strong reduction of the barrier among equivalent minima and T-t on going from KCl to NaCl. The optical absorption spectrum of KCl:Ag2+ is also explained through present calculations. (c) 2006 Elsevier B.V. All rights reserved.