The Htickel MO Method

The extent of approximation and parameterization varies with the different MO methods. As computer power has expanded, it has become possible to do MO calculations on larger molecules and with larger basis sets and fewer approximations and parameters. The accuracy with which calculations can predict structure, energy, and electron density has improved as better means of dealing with the various approximations have been developed. In the succeeding sections, we discuss three kinds of MO calculations (1) the Htickel MO method, (2) semiempirical methods, and (3) ab initio methods, and give examples of the application of each of these approaches. 

The quantity a is called the Coulomb integral, it represents the binding of an electron in a 2p orbital and is considered to be constant for all sp carbon atoms. The 

The equation for the HMOs of completely conjugated monocyclic polyenes is 

What do we learn about molecules such as 1,3,5-hexatriene and benzene from the HMO description of the tt orbitals  

The frontier MOs are identified and described. The frontier orbitals are the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO). These orbitals are intimately involved in chemical reactivity, because they are the most available to electrophiles and nucleophiles, respectively. From the atomic coefficients, which can be represented graphically, we see the symmetry and relative atomic contribution 

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