  Dividing the lattice into blocks with singlet ground state and knowing the
exact low energy spectrum of the blocks and of dimers (or trimers) of blocks,
it is possible to approach the lowest part of the lattice spectrum through an
excitonic type effective model. The potentialities of the method are
illustrated on the 1-D frustrated chain and the 1/5-depleted square and the
plaquette 2-D lattices. The method correctly locates the phase transitions
between gapped and non-gapped phases.
