Model Hamiltonians derived from Kohn-Sham theory

We discuss a general way to derive approximate molecular orbital (MO) metho ds starting from some reference MO theory. Ln particular, we present a mode l Hamiltonian that is based on a Kohn-Sham reference and that is free of ad justable parameters. This Hamiltonian is a Linear combination of atom-cente red ket-bra operators, each of which is easily derived from the results of Kohn-Sham atomic calculations. The resulting equations are similar to those of extended Huckel (eH) theory and are as efficient computationally as eH. Orbital energies for a few small molecules show that this method is more s table with respect to choice of basis set, and slightly more accurate, than eH. We improved the accuracy of our model Hamiltonian by introducing param eters fitted to the higher level of theory. These parameters define a basis of pseudoatomic orbitals that are, in a certain sense, optimal for the mol ecule used in the fitting procedure. We illustrate our method by calculatin g the eigenvalue spectrum of silicon clusters. (C) 2000 John Wiley & Sons, Inc.