The Extended Huckel Method (EHM), as any other theoretical model, has its strengths and weaknesses. Its major strength would lie in its capacity to address very complex geometries. It can determine the relative energy of different geometrical configurations and it gives a good qualitative picture of the molecular orbitals. But it can be very poor at predicting energy differences between isomers or even 100% correct geometries. Charge differences, particularly between atoms of very different electronegativity, can be grossly exaggerated.

Thus, it is common in many theoretical studies to use the Extended Hückel molecular orbitals as a preliminary step to determining the molecular orbitals by a more sophisticated method such as the CNDO (Complete Neglect of Differential Overlap) method and ab-initio methods.

As far Nt_STM is concerned, we implemented EHM because it is the most commonly used theoretical model in the field, despite the limitations above. Nonetheless, alternative theoretical models will be integrated in the future versions.