Ab initio theoretical calculations on molecular models of the short range structure of amorphous mixed MPON (M=Al, Ga, In, and Tl) oxynitridcs are presented. Theoretical results are used to correlate the changes observed in experimental DRIFTS spectra of precursor oxides (AlPO and AlGaPO solids), as well as oxynitridcs (AlPON and AlGaPON solids) with structural modifications in those solids. First principles DFT calculations on these model systems allow the study, from a thermodynamic viewpoint, of the extent and nature of N/O substitution on AlPON and mixed MAlPON solids as well as the relative stability of different nitrided species. It is also shown how computer simulations, using models which have been previously validated, allow information to be extracted concerning structure and properties of behaviour of chemical systems in a manner which allows rationalisation of the observed experimental behaviour and concentrate research effort in a given direction.