  Given the ground state wavefunction for an interacting lattice model, we
define a "correlation density matrix"(CDM) for two disjoint, separated clusters
$A$ and $B$, to be the density matrix of their union, minus the direct product
of their respective density matrices. The CDM can be decomposed systematically
by a numerical singular value decomposition, to provide a systematic and
unbiased way to identify the operator(s) dominating the correlations, even
unexpected ones.
