Exploring new construction schemes for extended-hierarchy CI (ehCI) wavefunctions

Following the work of Loos et al. on hierarchy CI (hCI) we propose an extended way of partitioning the Hilbert space by combining the excitation and the seniority sectors in a more general way. We define the hierarchy parameter, h, involving two “weights” (alphas), measuring the importance of excitation (e) and seniority (s) contributions to the wavefunction according to: h = α1.e + α2.s. We test these new CI methods on the dissociation of chemical systems and analyze them based on the importance of these contributions.