Hierarchy Configuration Interaction: Combining Seniority\nNumber and Excitation Degree
Fábris Kossoski, Yann Damour, Pierre‐François Loos
Abstract
We propose a novel\npartitioning of the Hilbert space, hierarchy\nconfiguration interaction (hCI), where the excitation degree (with\nrespect to a given reference determinant) and the seniority number\n(i.e., the number of unpaired electrons) are combined in a single\nhierarchy parameter. The key appealing feature of hCI is that each\nhierarchy level accounts for all classes of determinants whose number\nshares the same scaling with system size. By surveying the dissociation\nof multiple molecular systems, we found that the overall performance\nof hCI usually exceeds or, at least, parallels that of excitation-based\nCI. For higher orders of hCI and excitation-based CI, the additional\ncomputational burden related to orbital optimization usually does\nnot compensate the marginal improvements compared with results obtained\nwith Hartree–Fock orbitals. The exception is orbital-optimized\nCI with single excitations, a minimally correlated model displaying\nthe qualitatively correct description of single bond breaking at a\nvery modest computational cost.