where n o and s refer to the number of singly occupied orbitals and the target spin, respectively. For the (20, 20) active space, Ω CSF attains its maximum at s = 2. The multi-reference character in a CSF basis is amplified when transformed to the corresponding SD basis, and more walkers are required within the FCIQMC algorithm for a correct stochastic representation of the more complex wave function. Therefore, while it is possible to calculate pure spin states for such systems using an SD basis, the number of walkers required for converging the FCIQMC dynamics is far larger than in a CSF basis. While the reference calculation in a spin-pure basis was converged with N w = 1 • 10 6 walkers, the SD based calculations used N w = 5 • 10 7 walkers and was not fully converged in all spin states, as shown by the difference in Figure . We would like to stress that the SD based dynamics were not at the computational limits and increasing the walker number to achieve convergence in the SD based FCIQMC would have been possible but expensive. This was not done as it was not in the scope of the present investigation. The main goal was to compare the truncation effects of GAS compared to CAS.