Abstract
In a recent paper of this journal ( Tajti ; ; Szalay . J. Chem. Theory Comput. 2019 , 15 , 5523 ), we have shown that failures of the CC2 method to describe Rydberg excited states as well as potential energy surfaces of certain valence excited states can be cured by spin-component scaled (SCS) versions SCS-CC2 and SOS-CC2 to a large extent. In this paper, the related and popular second-order algebraic diagrammatic construction (ADC(2)) method and its SCS variants are inspected with the previously established methodology. The results reflect the similarity of the CC2 and ADC(2) models, showing identical problems in the case of the canonical form and the same improvement when spin-component scaling is applied.
Highlights
In two recent papers[1,2] in this journal, we have extended the large pool of benchmark calculations on vertical excitation energy[3−14] with characterization of the excited state potential energy surfaces using coupled-cluster (CC) type methods
In our recent study,[2] we investigated spin-component scaled versions of CC221,22 and found that both the scaled-opposite-spin (SOS-CC2) and spin-component scaled (SCS-CC2) variants cure the problem of CC2 to a considerable extent and the resulting potential energy surfaces run quite close to those obtained with CCSD
Statistics on the error of the vertical excitation energies are presented in Table 1 for ADC(2) and its spin-component scaled variants, evaluated against CC3 reference values
Summary
In two recent papers[1,2] in this journal, we have extended the large pool of benchmark calculations on vertical excitation energy[3−14] with characterization of the excited state potential energy surfaces using coupled-cluster (CC) type methods. The parent CCSD method (termed either EOM-CCSD16−18 or CCSD-LR19,20) gives usually reasonable results, but the inclusion of approximate triple excitations is necessary to achieve high accuracy. Since this is generally not possible for larger systems, improved second-order approximations are requested.[12,13] Along this line, in our recent study,[2] we investigated spin-component scaled versions of CC221,22 and found that both the scaled-opposite-spin (SOS-CC2) and spin-component scaled (SCS-CC2) variants cure the problem of CC2 to a considerable extent and the resulting potential energy surfaces run quite close to those obtained with CCSD.
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