2021/06/30 by Antoine Marie, Fábris Kossoski, Pierre-François Loos · 1 citation
Physics and Astronomy · #Advanced Chemical Physics Studies #Atomic orbital #Cold Atom Physics and Bose-Einstein Condensates #Configuration interaction #Coupled cluster #Excitation #Excited state #Operator (biology) #Slater determinant #Spectroscopy and Quantum Chemical Studies #Square (algebra) #Symmetry (geometry) #cond-mat.mtrl-sci #cond-mat.str-el #nucl-th #physics.chem-ph #physics.comp-ph
paper · pdf · doi:10.1063/5.0060698
published as J. Chem. Phys. 155, 104105 (2021) · 16 pages, 8 figures
openalex created_date 2021/07/05 · arxiv created 2021/08/20 · openalex publication_date 2021/09/09 · arxiv updated 2021/09/10 · openalex updated_date 2026/08/05
In single-reference coupled-cluster (CC) methods, one has to solve a set of non-linear polynomial equations in order to determine the so-called amplitudes that are then used to compute the energy and other properties. Although it is of common practice to converge to the (lowest-energy) ground-state solution, it is also possible, thanks to tailored algorithms, to access higher-energy roots of these equations that may or may not correspond to genuine excited states. Here, we explore the structure of the energy landscape of variational CC and we compare it with its (projected) traditional version in the case where the excitation operator is restricted to paired double excitations (pCCD). By investigating two model systems (the symmetric stretching of the linear H4 molecule and the continuous deformation of the square H4 molecule into a rectangular arrangement) in the presence of weak and strong correlations, the performance of variational pCCD (VpCCD) and traditional pCCD is gauged against their configuration interaction (CI) equivalent, known as doubly occupied CI, for reference Slater determinants made of ground- or excited-state Hartree–Fock orbitals or state-specific orbitals optimized directly at the VpCCD level. The influence of spatial symmetry breaking is also investigated.