ELECTRON PAIRS IN THE BERYLLIUM ATOM¹

Seniority-based coupled cluster theory

“…One can test this by defining configuration interaction (CI) restricted to the zero seniority sector of Hilbert space, which we refer to as doubly occupied configuration interaction (DOCI). [11][12][13][14][15][16] Because DOCI is not invariant to the orbitals with respect to which seniority is defined, we optimize this choice energetically. This is analogous to optimizing the identity of the reference determinant in an excitation-truncated CI calculation, or to optimizing the orbitals in CAS-SCF, though DOCI is generally size consistent.…”

The influence of orbital rotation on the energy of closed-shell wavefunctions

“…[7][8][9][10] Later on, the method was abandoned due to its factorial scaling until it recently experienced a revival for various reasons, be it as a benchmark for other closed-shell methods or as a starting point for even more general wavefunctions. [6,[11][12][13][14][15] It was realized that DOCI is a particular rung on the ladder leading from HF to full CI: it is the most elaborate closed-shell wavefunction possible, and at the same time also the lowest level in a hierarchical concept called seniority, which stepwise leads to full CI.…”

On a new expansion of wave functions of 2N-electron systems

“…Calculations on some four-electron systems have shown that a simple antisymmetrized product of strongly orthogonal geminals is a fairly good approximation for the ground state [1,2,3]. Similar result may be expected for more complex systems consisting of spatially localized two-electron groups.…”