Strengths and Weaknesses of the Simple Hiickel Method

The defects of the SHM arise from the fact that it treats only n electrons, and these only very approximately. The basic Hiickel method described here has been augmented in an attempt to handle non-re substituents, e.g. alkyl groups, halogen groups, etc., and heteroatoms instead of carbon. This has been done by treating the substituents as n centers and embodying empirically altered values of a and jl. so that in the Fock matrix values other than —1 and 0 appear. However, the values of these modified parameters that have been employed vary considerably [54], which tends to diminish one s confidence in their reliability. 

The approximations in the SHM are its peremptory treatment of the overlap integrals S (Section 4.3.4, discussion in connection with Eqs. 4.55), its drastic truncation of the possible values of the Fock matrix elements into just a, jl and 0 (Section 4.3.4, discussion in connection with Eqs. 4.61), its complete neglect of electron spin, and its glossing over (although not exactly ignoring) interelectronic repulsion by incorporating this into the a and jl parameters. 

The overlap integrals S are divided into just two classes  

Setting the Fock matrix elements equal to just a, /J and 0 Setting / j)iH pjdv = Hij = a, /J or 0 

7 The Determinant Method of Calculating the Hiickel c s and Energy Levels 

---