In order to contrast the HMO theory (HMO) with a simple valence bond theory (Heisenberg-Dirac model), which are two opposite asymptotic limit cases of the Hubbard model, Hubbard problems have been solved for four sites systems with four electrons. The Hueckel-Hubbard correlation diagrams, where employs a parameter: U/t, have been obtained for these system. From correlation diagrams some interested results have been deduced in detail.e

Key words: CONJUGATION, ELECTRONIC STATES, BONDING, IRREDUABLE REPRESENTATION, CORRELOGRAM, MOLECULAR MODEL, HUCKEL MOLECULAR ORBITAL