The results are reported of simulations on a number of models of modification to the canonical inhomogeneous (in x) coupled wave model (uniform coupling amplitude gamma and mismatch wave vector gradient k'). It is concluded that gamma -1 mod d2 gamma /dx2 mod =ly-2 defines a useful gamma curvature ly-2 which must be above a minimum value for instability, and suggested that a similar curvature criterion may well apply for nonuniform k'(x) with uniform gamma . The transition between gamma slabs and previous smooth humped gamma forms is bridged using a hyper-Lorentzian gamma = gamma 0(1+(x2/lgamma 2)m)-1.