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.