Austin Model limitations

Various parameterizations of NDDO have been proposed. Among these are modified neglect of diatomic overlap (MNDO),152 Austin Model 1 (AMI),153 and parametric method number 3 (PM3),154 all of which often perform better than those based on INDO. The parameterizations in these methods are based on atomic and molecular data. All three methods include only valence s and p functions, which are taken as Slater-type orbitals. The difference in the methods is in how the core-core repulsions are treated. These methods involve at least 12 parameters per atom, of which some are obtained from experimental data and others by fitting to experimental data. The AMI, MNDO, and PM3 methods have been focused on ground state properties such as enthalpies of formation and geometries. One of the limitations of these methods is that they can be used only with molecules that have s and p valence electrons, although MNDO has been extended to d electrons, as mentioned below. 

The model used here is relatively simple, but the results make sense given our understanding of controls on decomposition and mineralization in ecosystems. Also, more complex models yield similar results. Year-to-year variations in NPP in Century are controlled by interactions between precipitation and precipitation-induced variations in N mineralization (Burke et al, 1997) these year-to-year variations can drive losses of N even when it limits NPP in most years. The Pnet model also predicts year-to-year variations in nitrate leaching from deciduous forest watersheds as a consectuence of variations in precipitation these predictions are strongly supported by watershed-level observations (Aber and Driscoll, 1997). This mechanism could help to explain observations that the N cycle appears to be more open in semiarid areas than in mesic forest ecosystems, in the sense that both inputs and outputs of N are larger relative to N pools within ecosystems (Austin and Vitousek, 1998). 

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