, while for ∆ = S the adaptive shift reduces to the conventional i-FCIQMC algorithm. Lowering the offset gives higher total energy estimates, using the same number of walkers. Plotting the energy as a function of the number of walkers, there is a strict ordering between the energy curves for different offsets, with some converging from below (high offsets) while others from above (low offsets). By varying the offset, one can use this property to bracket the exact energy between the curves of different offsets (see Figure ). A good starting point for varying the offset is setting it to half the correlation energy ∆ = Ẽcorrelation /2. This estimate of the correlation energy, Ẽcorrelation , can be approximated by the shift, S, of an earlier FCIQMC calculation or by other methods like MP2 or coupled cluster calculations. For the test case applications investigated in this work the choice of ∆ = S/2 was already satisfactory for largely curing the initiator bias.