A systematic construction of configuration interaction wavefunctions in the complete CI space

We introduce a systematic approach to construct configuration interaction (CI) wavefunctions through a variant of the Monte Carlo CI (MCCI) method termed systematic-MCCI. Within this approach, the entire interacting space is systematically considered in batches, with the most important configurations across all batches becoming potential additions to the wavefunction. We compare this method to MCCI and a novel pruned-full configuration interaction (FCI) approach. For the ground state of neon, as described by the cc-pVTZ basis, we observe no apparent difference between systematic-MCCI, pruned-MCCI, and MCCI, with all recovering 99% of the correlation energy and producing a very similar wavefunction composition. We then consider the potential energy surface corresponding to the symmetric double hydrogen dissociation of water within a cc-pVDZ basis. Once again MCCI performs comparably to the systematic approaches. Despite systematic-MCCI having longer run times across the number of processors considered, we do observe very good scalability. We then extend this comparison to the first A₁ excited energy of carbon monoxide using the cc-pVDZ basis where the MCCI methods perform similarly, approximating this aforementioned energy to within 0.1 eV despite vast reduction in the wavefunction size. Finally, we consider the chromium dimer with the cc-pVTZ basis and 18 frozen orbitals. Here, we find that the systematic approach avoids being trapped in the same local minimum of configuration space as MCCI, yet MCCI can reach a lower energy by repeating the calculation with more processors.