determinants, and then compute the natural orbitals of this variational wavefunction. A subsequent CIPSI calculation is then performed in this natural orbital basis up to at least 5 × 10 7 determinants. The CIPSI values we report herein are obtained from a non-linear extrapolation of E (0) as a function of E P T 2 , as the latter quantity goes to zero. In all cases except H 20 at 2Å, the extrapolated value from the non-linear fit agreed with that from a 3-point linear extrapolation. Further details of the CIPSI calculations are provided in the Supplementary Information.