For the H 19 and H 20 ring systems that were beyond the reach of FCI, we have extrapolated selected Configuration Interaction energies 90 to estimate the FCI limit using Quantum Package 2.0, which implements the Configuration Interaction using a Perturbative Selection made Iteratively (CIPSI) algorithm. The CIPSI algorithm iteratively explores relevant subspaces of the N -electron Hilbert space by adding to a subspace only those determinants associated with an energy estimate greater than 10 -4 Hartrees from second-order perturbation theory (PT2). After each selection iteration, Ĥ is diagonalized within the expanded subspace. The variational energy, E (0) , is expected to converge with respect to the total number of determinants N dets in the subspace. Following the procedure outlined by Loos et al., we first perform a CIPSI calculation with HF orbitals up to at least 10 7