Coupled Cluster Valence Bond Method: Efficient Computer Implementation and Application to Multiple Bond Dissociations and Strong Correlations in the Acenes

Abstract

We describe an efficient implementation of the coupled cluster valence bond (CCVB) model. CCVB captures a certain essential part of the description of molecules with strong correlations (SC), which allows it to achieve correct energy profiles when covalent bonds are broken, while maintaining proper spin symmetry and size extensivity. To illustrate treatment of SC in bond breaking, we examine the symmetric dissociation of the sulfur allotropes S6 and S8 into triplet S atoms. To show applicability to larger systems and to explore whether CCVB can capture aspects of SC that arise in extended π systems, we report results for a series of acenes up to 12 fused benzene rings, with active spaces of up to 228 correlated electrons. The lowest-energy CCVB solutions found for two of the largest acenes show signatures consistent with multi-electron SC and partial delocalization.

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry

“…A different, computationally feasible approach suitable for strongly-correlated systems uses seniority-zero wavefunctions to describe the static/nondynamic part of the electron correlation en-ergy. [31][32][33][34][35][36][37][38][39][40][41][42][43][44] The missing dynamic electron correlation effects are included a posteriori in these ansätze using, for instance, many-body perturbation theory [45][46][47], coupled-cluster theory [48][49][50][51][52], extended random phase approximation [53], and density functional theory (DFT) corrections [54,55].…”

Orbital optimisation in the perfect pairing hierarchy: applications to full-valence calculations on linear polyacenes

“…Famously, DMRG calculations have been used to show that the larger linear polyacenes exhibit strong correlation for their π electrons [55]. Other approaches used have included density functional theory [56][57][58][59][60][61], multiconfiguration pair-density functional theory [62], projected Hartree-Fock theory [63], the random phase approximation [64], configuration interaction [60] (CI), adaptive CI [27], GW theory [65], variational two-electron reduced density matrix (VRDM) theory [66][67][68][69], Møller-Plesset perturbation theory [61,[70][71][72][73], spin-flip methods [74,75], CAS-SCF [53,59,71,72,76] as well as restricted-active space self-consistent field theory [77], valence bond [78] and CC valence bond (CCVB) theory [79,80], CC theory [70][71][72]80] and multireference averaged quadratic CC theory [81,82], an algebraic diagrammatic construction scheme [83], as well as PPH methods with approximate orbitals [50].…”

“…As far as we are aware, the only full-valence calculations on large polyacenes employing a method potentially capable of accurate treatment of strong correlation that have been published are the CCVB calculations performed contemporaneously in our group [79,80]. Thanks to our recent implementation [50], even the larger polyacenes can be treated accurately in full-valence active spaces using PPH methods, yielding an independent way to check the correctness of the recent CCVB results.…”

Spin Symmetry and Size Consistency of Strongly Orthogonal Geminals

“…Closing the introduction, let us note that triplet geminals can be incorporated in a spinpure manner by multireference coupled cluster [43][44][45] …”

“…The Coupled Cluster Valence Bond (CCVB)approach of Small et al45 is particularly relevant in the present context as it resorts solely to pairs of intra-geminal triplet states, bringing in essentially the same effect as EAPSG. There are N g (N g − 1)/2 extra parameters considered in CCVB in addition to those in APSG, N g standing for the number of geminals, and two orbitals being assigned to each geminal.…”