Title: Influence of broken-pair excitations on the exact pair wavefunction,
Abstract: Doubly occupied configuration interaction (DOCI), the exact diagonalization
of the Hamiltonian in the paired (seniority zero) sector of the Hilbert space,
is a combinatorial cost wave function that can be very efficiently approximated
by pair coupled cluster doubles (pCCD) at mean-field computational cost. As
such, it is a very interesting candidate as a starting point for building the
full configuration interaction (FCI) ground state eigenfunction belonging to
all (not just paired) seniority sectors. The true seniority zero sector of FCI
(referred to here as FCI${}_0$) includes the effect of coupling between all
seniority sectors rather than just seniority zero, and is, in principle,
different from DOCI. We here study the accuracy with which DOCI approximates
FCI${}_0$. Using a set of small Hubbard lattices, where FCI is possible, we
show that DOCI $\sim$ FCI${}_0$ under weak correlation. However, in the strong
correlation regime, the nature of the FCI${}_0$ wavefunction can change
significantly, rendering DOCI and pCCD a less than ideal starting point for
approximating FCI.