Separation of strong (bond-breaking) from weak (dynamical) correlation

We start from a coupled cluster ansatz, i.e. an exponential form e(S + T) for the wave operator. We decompose the cluster amplitude (S + T) into a part S which describes strong (bond-breaking) correlation and a part T for weak correlation. Strong correlation is taken care of by the APSG (antisymmetrized product of strongly orthogonal geminals) wave function (in intermediate normalization), to infinite order, i.e. variational and without any truncation. On top of the APSG function the weak (dynamic) correlation is treated by traditional coupled-cluster theory.