  We extend the spectral projectors method to staggered fermions. Applying the
index theorem to the staggered Dirac operator it is possible to work out an
expression for the topological susceptibility which depends only on the
orthogonal projectors on quasi zero-modes, as it has already been done for
Dirac-Wilson fermions. Besides, we generalize this method deriving analogous
expressions for all higher-order coefficients in the $\theta$-expansion of the
vacuum energy.
