It is seen from these expressions that, on taking into account the vibronic coupling, F ≠ ≠ 0 , the two APES curves change in different ways: in the upper sheet the curvature (the coefficient at Q 2 in the expansion on Q ) increases, whereas in the lower one it decreases. But until F 2 / K 0 < Δ Δ the minima of both states correspond to the point Q = 0 , as in the absence of vibronic mixing. However, if