  The Goldstone-Brueckner perturbation theory is extended to incorporate in a
simple way correlations associated with large amplitude collective motions in
nuclei. The new energy expansion making use of non-orthogonal vacua still
allows to remove the divergences originating from the hard-core of the bare
interaction. This is done through the definition of a new Brueckner matrix
summing generalized Brueckner ladders. At the lowest-order, this formalism
motivates variational calculations beyond the mean-field such as the Generator
Coordinate Method (GCM) and the Projected Mean-Field Method from a perturbative
point of view for the first time. Going to higher orders amounts to incorporate
diabatic effects in the GCM and to extend the projection technique from product
states to well-defined correlated states.
