Parametric approximation analog

Analogous to the PPP method for planar 7r-systems, semiempirical all-valence methods can be and were extended to include Cl, thus giving rise to a family of procedures based on the CNDO, INDO and NDDO variants of the zero-differential overlap (ZDO) approximation, many of which were applied also to the discussion of Cl effects in radical cations. Due to the parametric incorporation of dynamic correlation effects, such procedures often yield rather accurate predictions of excited-state energies and they continue to be the methods of choice for treating very large chromophores which are not amenable to ab initio calculations. 

Dimensionally, Pni defined by (162) is a collinear-like vibrational transition probability [cf. (144)] which depends parametrically on the initial conditions of the other degrees of freedom. The analogy to a collinear collision is purely formal, however, for there are no dynamical approximations which have been introduced the only approximations involved, beyond that of classical S-matrix theory itself, are the neglect of interference terms between different trajectories that contribute to the same S-matrix element and the assumption... 

---