For completeness, we also consider the effect of increasing the average bond strength by 25% and increasing the mass by 50%. In the first case, at small ∆ the v a is larger, as expected, but around ∆ ∼ 0.45 the group velocity falls below that of the first model with k = g = 1. This example further shows that large bonding inhomogeneity can potentially offset stronger (average) bonding to produce overall lower group velocities. In the second case, the ν a are simply scaled down by √ 1.5, which shows how increasing the (average) mass also reduces the group velocities.