Of particular interest is also the speed of convergence of the FCIQMC dynamics (in the CAS form) depending on the orbital basis. Only non-invariant orbital rotations are performed in the CASSCF procedure (inactive ↔ active, inactive ↔ virtual and active↔ virtual rotations). Thus, the active orbitals are not rotated among each other by the SCF proce- For GASSCF, the diagonalization of the 1-RDM in the full active space is not an invariant rotation. Instead, invariant is the rotation to pseudo-natural orbitals, defined as those which diagonalize each GAS subspace separately. While pseudo-natural orbitals do not disrupt the fragment-localized structure of the MO basis, the natural orbitals are in general delocalized across the entire system. The locality of correlation (within the fragment) is lost when the more delocalized natual orbitals are utilized, and the many-body wave function generally becomes more dense. FCIQMC dynamics are sensitive to the MO basis adopted. This argument has been discussed for exchange-coupled transition metal clusters. The benzene stack example shows how the MO representation can affect FCIQMC dynamics for weakly interacting closed-shell systems. The FCIQMC (in CAS form) projected energy, E proj , as a function of the walker population, and using both natural orbitals and fragment-localized orbitals is depicted in Figure . The FCIQMC dynamics (in CAS form) converge faster when fragment-localised molecular orbitals are utilized, rather than for example the corresponding more delocalized natural orbitals. This result represents the numerical evidence that fragment-localized orbitals produce sparser wave functions which are simpler to describe by a finite distribution of stochastic walkers. Within GASSCF, where the pseudo-natural orbitals are used, the convergence speed does not deteriorate, owing to the fact that pseudo-natural orbitals preserve the fragment-localized nature of the molecular orbitals. We also note, that the time-step can be chosen larger for fragment-localised orbitals