The extended Hückel-type covalent bond energy method prominently appears in many SQM methods. SQM methods compute approximate electronic structure energies efficiently enough to be suitable for large-size systems (e.g., biochemistry, material science, macromolecules). The general formalism is similar to ab initio methods except that integrals are reduced drastically by implementing and parametrizing analytical expressions to fit accurate thermochemical data. Many chemical applications use SQM methods extensively for qualitative studies. The quantitative results of these methods, in general, are unreliable for compounds distinct from the training set from which parameters were obtained. This indicates that SQMs are not reliable for extrapolating to large-scale systems.