Equation of motion coupled cluster method for electron attachment

Abstract

The electron attachment equation of motion coupled cluster (EA-EOMCC) method is derived which enables determination of the various bound states of an (N+1)-electron system and the corresponding energy eigenvalues relative to the energy of an N-electron CCSD reference state. Detailed working equations for the EA-EOMCC method are derived using diagrammatic techniques for both closed-shell and open-shell CCSD reference states based upon a single determinant. The EA-EOMCC method is applied to a variety of different problems, the main purpose being to establish its prospects and limitations. The results from EA-EOMCC calculations are compared to other EOMCC approaches, starting from different reference states, as well as other theoretical methods and experimental values, where available. We have investigated electron affinities for a wide selection of both closed-shell and open-shell systems. Excitation spectra of atoms and molecules with an odd number of electrons are obtained, taking the closed-shell ground state of the ion as a reference in the EA-EOMCC calculation. Finally we consider excitation spectra of some closed-shell systems, and find in particular that the electron attachment approach is capable of yielding accurate triplet excitation energies in an efficient way.

A quantum chemical study from a molecular transport perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions

“…(iv) Equation-of-motion (EOM) coupled-cluster (CC) approaches at singles and doubles (CCSD) 36 , hybrid (CC2) 37 , and perturbative [CCSD(2)] 38,39 levels will be extensively applied in this paper. Corrections due to triples [EOM-CCSD(T)] will be also considered; the fact that these corrections are altogether negligible is an indication on the accuracy of the state-of-the-art EOM-CCSD method.…”

“…The inspection of the tables presented below reveals that the results obtained via the state-of-the-art IP-and EA-EOM-CCSD method 36,45 and aug-cc-pVDZ (Dunning augmented correlation consistent double zeta) sets can be trusted. This is illustrated both by the good agreement between the EOM-CCSD and the ∆-CCSD values and by the fact that corrections due to triples [CCSD(T)] yield changes that are irrelevant within numerical errors.…”

The effect of microhydration on ionization energies of thymine

“…As such, they can be described accurately and efficiently by the ionization potential (IP) and electron affinity (EA) variants of equation-ofmotion coupled-cluster (EOM-CC) methods [27][28][29][30]. EOM-IP, which relies on the N-electron closed-shell reference, is free from the symmetry breaking and spin-contamination problems that are ubiquitous in open-shell calculations, and is capable of describing charge localization patterns in ionized clusters [4,7,23,31].…”

Double, Rydberg and charge transfer excitations from pairing matrix fluctuation and particle-particle random phase approximation

“…For example, the spin-flip (SF-) EOM-CC 19,20 method uses an N -electron high-spin triplet reference, the ionization-potential/electron-attachment (IP/EA-) EOM-CC 21,22 use (N ± 1)-electron ground states, the double ionization-potential/electron-attachment (DIP/DEA-) EOM-CC 23,24 use (N ±2)-electron ground states and the triple ionization-potential/electronattachment (TIP/TEA-) EOM-CC 25 can use (N ± 3)-electron ground states. Such EOM-CC methods involving electron number changes have roots in Fock-Space coupled cluster (FSCC) theory [26][27][28][29][30][31][32][33] and similarity transformed EOM-CC 34,35 .…”