A noniterative perturbative triples correction for the spin-flipping and spin-conserving equation-of-motion coupled-cluster methods with single and double substitutions

Abstract

A noniterative N (7) triples correction for the equation-of-motion coupled-cluster method with single and double substitutions (CCSD) is presented. The correction is derived by second-order perturbation treatment of the similarity-transformed CCSD Hamiltonian. The spin-conserving variant of the correction is identical to the triples correction of Piecuch and co-workers [Mol. Phys. 104, 2149 (2006)] derived within method-of-moments framework and is not size intensive. The spin-flip variant of the correction is size intensive. The performance of the correction is demonstrated by calculations of electronic excitation energies in methylene, nitrenium ion, cyclobutadiene, ortho-, meta-, and para-benzynes, 1,2,3-tridehydrobenzene, as well as C-C bond breaking in ethane. In all cases except cyclobutadiene, the absolute values of the correction for energy differences were 0.1 eV or less. In cyclobutadiene, the absolute values of the correction were as large as 0.4 eV. In most cases, the correction reduced the errors against the benchmark values by about a factor of 2-3, the absolute errors being less than 0.04 eV.

Singlet stabilization of oxazole- and isoxazolenitrene-n-oxides by radical delocalization

“…Geometries of the triplet nitrenes were optimized at the UCCSD/6-31G * level. Optimization of the open-and closed-shell singlet state geometries was carried out by using the spin flip method, at the SF-CCSD/6-31G * level of theory [41,42]. CCSD geometry optimizations were all carried out using unrestricted Hartree-Fock orbitals of the triplet state as the reference.…”

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

“…75 have to be used instead of aug-cc-pVX Z to obtain converged energies for the parent Rydberg states and moreover, the CAP onset has to be chosen much larger than the spatial extent of the neutral ground state and also that of the parent states in order to limit the perturbation of the latter states to an acceptable level. EOM-EA-CCSD* and EOM-EA-CCSD(fT), which correct only target states, do not yield accurate attachment energies, although significant improvements over EOM-CCSD have been reported for vertical ionization potentials 49 and energy differences between EOM-SF states 48 . In contrast, the EOM-EA-CCSD(T)(a)* method, in which the CCSD reference state is corrected for the effect of triples excitations before constructing the similaritytransformed Hamiltonian, reliably improves upon EOM-EA-CCSD attachment energies and is comparable in accuracy to EOM-EA-CC3.…”

“…There have also been many efforts [41][42][43][44][45][46][47][48][49][50] to devise EOM-CC methods that account for the effect of triples excitations in a non-iterative fashion similar to CCSD(T) 51,52 for groundstate energies and properties. It has become apparent, however, that a balanced correction of several states is more difficult to achieve than that of a single state.…”

“…As for the first objective, EOM-CCSD* 44,45 and EOM-CCSD(fT) 48,49 are investigated as examples of methods that correct only the target state and the novel EOM-CCSD(T)(a)* method 50 is considered because it includes a correction to the CCSD reference state as well. As for the second objective, different effects can be anticipated for shape and Feshbach resonances.…”

Heterogeneously Substituted Radicals and Carbenes: Photoelectron Imaging of the FC(H)CN– and FCCN– Anions

“…The VDEs of the anion were also determined using the EOM-IP-CCSD(dT) methodology 29,33 (IP, for brevity). In these calculations, the singlet ( 1 A′: ...σ 2 π 0 ) and triplet ( 3 A″:…”