Variational coupled cluster for ground and excited states

In single-reference coupled-cluster (CC) methods, one has to solve a set of non-linear polynomial equations in order determine the so-called amplitudes which are then used compute energy and other properties. Although it is common practice converge (lowest-energy) ground-state solution, also possible, thanks tailored algorithms, access higher-energy roots these may or not correspond genuine excited states. Here, we explore structure landscape variational CC (VCC) compare with its (projected) traditional version (TCC) case where excitation operator restricted paired double excitations (pCCD). By investigating two model systems (the symmetric stretching linear \ce{H4} molecule continuous deformation square into rectangular arrangement) presence weak strong correlations, performance VpCCD TpCCD gauged against their configuration interaction (CI) equivalent, known as doubly-occupied CI (DOCI), for reference Slater determinants made ground- excited-state Hartree-Fock orbitals state-specific optimized directly at level. The influence spatial symmetry breaking investigated.