  We study the eigenstates of a paradigmatic model of many-body localization in
the Fock basis constructed out of the natural orbitals. By numerically studying
the participation ratio, we identify a sharp crossover between different phases
at a disorder strength close to the disorder strength at which subdiffusive
behaviour sets in, significantly below the many-body localization transition.
We repeat the analysis in the conventionally used computational basis, and show
that many-body localized eigenstates are much stronger localized in the Fock
basis constructed out of the natural orbitals than in the computational basis.
