On the Relation Between Semi-Empirical MO Methods and Rigorous Methods in Quantum Chemistry

Semi-empirical molecular orbital theory is explained in terms of the exact solution to the electronic Schrödinger equation. The necessity is pointed out for a parametrization which simulates the effect of a large orbital basis. The non-uniqueness of parameters is discussed using results obtained by Davidson and Ponec. The present work is meant as an introduction to an accompanying paper on the HAM/4 parametrization of atomic energies.