  Multi-configurational electronic structure theory delivers the most versatile
approximations to many-electron wavefunctions, flexible enough to deal with all
sorts of transformations, ranging from electronic excitations, to open-shell
molecules and chemical reactions. Multi-configurational models are therefore
essential to establish universally applicable, predictive ab initio methods for
chemistry. Here, we present a discussion of explicit correlation approaches
which address the nagging problem of dealing with static and dynamic electron
correlation in multi-configurational active-space approaches. We review the
latest developments and then point to their key obstacles. Our discussion is
supported by new data obtained with tensor network methods. We argue in favor
of simple electrons-only correlator expressions that may allow one to define
transcorrelated models in which the correlator does not bear a dependence on
molecular structure.
