The GAS strategy is first applied to an N 4 tetrahedron model system as a proof of concept. The smaller N 4 model is chosen to mimic the weak magnetic interactions across the four magnetic centers of the transition metal cubane. In the all-ferric Fe (III) 4 S 4 cubane, each magnetic center is in a local spin s loc = 5/2 with five unpaired electrons, for a total of 20 unpaired electrons. The (20, 20) active space is the smallest, that can be chosen to describe spin interactions in this system, which is already too large for conventional multiconfigurational techniques. The N 4 model is characterized by three unpaired electrons per site, a local spin s loc = 3/2, and a total of 12 valence electrons. Conventional CAS (12, 12) calculations are routinely feasible and fast and will be used as a reference for comparisons with GAS (12, 12) calculations, in Slater determinant and spin-adapted basis. Considering that the current implementation of the Stochastic-GAS operates on the basis of Slater determinants (SDs), the N 4 will also be used to address the question of whether spin-pure solutions can be obtained from our SD based Stochastic-GAS method. Starting from an SD as reference, the FCIQMC dynamics preserve the spin projection, m s , and convergence to the lowest spin state with s ≥ |m s | is to be expected. It follows that for antiferromagnetically coupled systems, it is possible to target spin pure states by adjusting the spin projection of our starting guess. However, for ferromagnets, where higher-spin means lower energy, any spin-unconstrained optimization will inevitably lead to the highspin ground state, independently of the initial choice of m s . The analysis of the following spin-systems is carried with this limitation in mind. (12, 12) was performed, the optimized natural orbitals were subsequently localised with the Pipek-Mezey method and used as starting orbitals for subsequent GAS-CI calculations. No SCF orbital optimizations have been carried for this system, as our main focus is the rationalization of electron correlation mechanisms that are missed with respect to the corresponding CAS, when interspace excitations are sevelery constrained by GAS. This comparison is only possible if the same active orbitals are utilized for CAS and GAS approaches, except for the invariant rotations among the active orbitals.