APPROXIMATE VB-MO SCHEMES FOR EXCITATION GAPS IN HUBBARD MODELS

The excitation gaps in the singlet and triplet manifolds for finite Hu bbard models in one, two and three dimensions have been obtained using different approximate configuration interaction (CI) schemes, as a fu nction of the correlation strength, by using valence bond (VB) functio ns constructed over the molecular orbital (MO) basis. These are compar ed with numerically exact results and it is found that the scheme in w hich all particle hole excitations below a given threshold are include d is the method of choice. The excitation energies are well reproduced , in trend as well as magnitude, particularly when the threshold equal s the bandwidth of the corresponding noninteracting system.