Fock matrix extended Huckel method

Pyykko and coworkers have formulated a relativistic extended Huckel method for molecules. This method incorporates relativistic effects by a systematic parametrization using Desclaux s atomic relativistic Dirac-Fock calculation. The Dirac-Fock atomic energies and the off-diagonal elements are proportional to the product of the overlap matrix element in the relativistic lsjm basis. Pyykko and coworkers - have applied this method to a number of molecules. 

The Huckel methods perform the parameterization on the Fock matrix elements (eqs. (3.50) and (3.51)), and not at the integral level as do NDDO/INDO/CNDO. This means that Huckel methods are non-iterative, they only require a single diagonalization of the Fock (Huckel) matrix. The Extended Huckel Theory (EHT) or Method (EHM), developed primarily by Hoffmann again only considers the valence electrons. It makes use of Koopmans theorem (eq. (3.46)) and assigns the diagonal elements in the F... 

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