The band-anti-crossing (BAC) model was originally introduced as a phenomenological method to describe the electronic structure of GaInNAs. We present here a Green's function model to derive explicitly the BAC model in ordered Ga(In)N(x)A(1-x) structures. The Green's function model is based on the tight-binding method, which we have used previously to confirm that N forms a resonant defect state above the conduction band edge in GaAs. By introducing the Green's function model we derive explicitly that the resonance becomes delocalised and spread over several energy states as x increases, but that the two-level BAC model still gives an excellent description of the conduction band edge in ordered supercells. We then extend the model to show that the conventional 8-band k (.) p Hamiltonian must be modified to include two extra spin-degenerate states, giving a 10-band model for Ga(In)NxAs1-x heterostructures. (C) 2002 Elsevier Science Ltd. All rights reserved.