Half-projection of doubly-occupied configuration interaction wave functions

JAVIER GARCIA; OFELIA B. OÑA; DIEGO R. ALCOBA; ALICIA TORRE; LUIS LAIN; GUSTAVO E. MASSACESSI

La Plata

Workshop; X Workshop on Novel Methods for Electronic Structure Calculations; 2023

Universidad Nacional de La Plata

The doubly-occupied configuration interaction (DOCI) method expresses the $N$-electron system wave function in terms of doubly occupied Slater determinant basis sets \cite{Bytautas_2011}. It has been proven that the DOCI method captures most of the static correlation and, consequently, it has been proposed as an alternative to excitation-based truncations of the full configuration interaction (FCI) treatment. The energies of the DOCI Hamiltonian depend on the single-particle basis set employed, and therefore a minimization with respect to orbital rotations is required. If this optimization is carried out by performing the same transformation on the $\alpha$- and $\beta$- orbitals (restricted DOCI, or RDOCI), one obtains a good description of molecules near equilibrium distances, but the dissociation products are not predicted correctly. On the other hand, if the optimization is performed by allowing the $\alpha$- and $\beta$- orbitals to be different (unrestricted DOCI, or UDOCI), the description of molecular dissociation, in general, is correct but at the cost of global spin contamination.