than for delocalized ones ( ∆τ i = 1.90 : FCIQMC projected energy, E proj , against the number of iterations using the more delocalized natural orbitals and the fragment-localized orbitals, and increaingly larger number of walkers. The system is the benzene stack (Figure ) with an inter-fragment distance of 6.0 Å. The fragment-localised orbitals utilized here are the CASSCF orbitals. The natural orbitals are obtained from the diagonalization of the corresponding one-body RDM.