Non-iterative triple excitations in equation-of-motion coupled-cluster theory for electron attachment with applications to bound and temporary anions

Abstract

The impact of residual electron correlation beyond the equation-of-motion coupledcluster singles and doubles approximation (EOM-CCSD) on positions and widths of electronic resonances is investigated. To establish a method that accomplishes this task in an economical manner, several approaches proposed for the approximate treatment of triples excitations are reviewed with respect to their performance in the electron attachment (EA) variant of EOM-CC theory.The recently introduced EOM-CCSD(T)(a)* method, which includes non-iterative corrections to the reference and the target states, reliably reproduces vertical attachment energies from EOM-EA-CC calculations with singles, doubles, and full triples excitations in contrast to schemes in which non-iterative corrections are applied only to the target states.

Green’s function coupled cluster formulations utilizing extended inner excitations

“…Generally speaking, the GFCC-i(n,m) formalism can be viewed as an extension of previous studies of approximate inclusion of higher-order excitations in the IP/EA-EOMCC methods represented by a broad class of iterative and non-iterative methodologies including EOMIP-CCSD * [74][75][76], CCSDT-n (n=1,2,3) [77][78][79][80][81], CCn (n=2,3) [82,83], EOM-CC(m,n) [84][85][86], and EOM-CCSD(T)(a) * formalisms [87,88]. Moreover, the EOM-CC(m,n) approach introduced by Hirata et al combines different levels of approximations for the ground-state CC Ansatz and equation-of-motion CC operators R K for vertical excitation energies, ionization potential, and electron affinities has already been applied to molecular systems.…”

Coupled-cluster techniques for computational chemistry: The CFOUR program package

“…24,189,193 For excited states that can be characterized as "single excitations", EOM-CCSD theory gives excitation energies that are usually no more than 0.25 eV in error, and tends towards overestimation. [199][200][201] Later developments led to EOM-CCSDT [202][203][204] and EOM-CCSDTQ, 205,206 as well as general arbitrary-order EOM-CC 47 29 shows considerable promise, 200,[219][220][221] and might be the method of choice for future applications.…”

The quest to uncover the nature of benzonitrile anion

“…The accuracy of EOM-CC can be systematically improved by including higherlevel of excitations in T and R, up to the exact limit. Here we account for the effect of triple excitations by using perturbative correction, i.e., the EOM-EA-CCSD(T)(a) * method 43,44 .…”

“…Thus, the energies of DBS were computed at the geometries of the neutral benzonitrile. Triples corrections to the VDE and AEA of the VA were computed with EOM-EA-CCSD(T)(a) * method 43,44 using aug-cc-pVTZ. We assume that the effect of triples cancels out for the DBS, because the unpaired electron does not participate in the bonding.…”