Multiple Solutions to the Single-Reference CCSD Equations for NiH

It is typically assumed that once a Hartree-Fock (HF) reference wave function is determined, the correlated wave function obtained from that HF wave function describes the same electronic state. In this paper, we report the appearance of multiple CCSD solutions obtained from the UHF reference wave function for the known ground state of a chemically interesting molecule, NiH. To determine a correspondence between the computed CCSD solutions and the physical electronic states, we consider several characteristics of the CCSD wave functions, e.g., potential energy curves, spin density isovalue plots, and excited state studies via EOM-CCSD calculations. Finally, the use of Brueckner orbitals is encouraged as a way to avoid some of the problems highlighted here for HF-based coupled cluster calculations in such challenging systems.