A New Benchmark Set for Excitation Energy of Charge Transfer States: Systematic Investigation of Coupled Cluster Type Methods

Abstract

The numerous existing publications on benchmarking quantum chemistry methods for excited states rarely include Charge Transfer (CT) states, although many interesting phenomena in, e.g., biochemistry and material physics involve the transfer of electrons between fragments of the system. Therefore, it is timely to test the accuracy of quantum chemical methods for CT states, as well. In this study we first propose a new benchmark set consisting of dimers having low-energy CT states. On this set, the vertical excitation energy has been calculated with Coupled Cluster methods including triple excitations (CC3, CCSDT-3, CCSD(T)(a)*), as well as with methods including full or approximate doubles (CCSD, STEOM-CCSD, CC2, ADC(2), EOM-CCSD(2)). The results show that the popular CC2 and ADC(2) methods are much less accurate for CT states than for valence states. On the other hand, EOM-CCSD seems to have similar systematic overestimation of the excitation energies for both types of states. Among the triples methods the novel EOM-CCSD(T)(a)* method including noniterative triple excitations is found to stand out with its consistently good performance for all types of states, delivering essentially EOM-CCSDT quality results.

Assessing challenging intra‐ and inter‐molecular charge‐transfer excitations energies with double‐hybrid density functionals

“…Such accurate ab initio reference data are mostly SCS-CC2 quality 86,87 or obtained using higher-order methods (e.g., CCSDT-3). The SCS-CC2 might be considered not accurate enough for benchmark studies; however, according to recent work, such values are sufficiently close to other more costly methods [88][89][90] and their use is not expected to significantly influence the general conclusions that are going to be reached.…”

A Simple Range-Separated Double-Hybrid Density Functional Theory for Excited States

“…The latter compilation, which is hereafter referred to as the SG set, comprises 32 valence (π → π*, n → π*, σ → π*), and 6 Rydberg transitions. The comprehensive CT benchmark set recently proposed by Szalay et al 112 contains 14 excitation energies for 8 molecular complexes, such as ammonia–fluorine, acetone–fluorine, pyrazine–fluorine, ammonia–oxygen-difluoride, acetone–nitromethane, ammonia–pyrazine, pyrrole–pyrazine, and tetrafluoroethylene–ethylene systems. The reference energies were calculated at the CCSDT-3 level using the cc-pVDZ basis sets.…”

“… Error measures for the calculated excitation energies for the CT test set 112 using the cc-pVDZ basis sets with the corresponding auxiliary bases. For clarity, the bars for the rightmost four methods are not proportional to the corresponding error measures, but the relevant values are displayed on the bars.…”

“…For clarity, the bars for the rightmost four methods are not proportional to the corresponding error measures, but the relevant values are displayed on the bars. The CCSD values were taken from ref ( 112 ).…”

“…The value of the parameters is greatly in line with the ground-state results of ref ( 71 ). Cross-validation was performed on several test sets using the default parameters, such as the benchmark compilation of Schwabe and Goerigk, 56 a CT set of Szalay and co-workers, 112 another CT set of Goerigk et al, 81 the widely used compilation of Thiel and co-workers, 113 a set on the two lowest excitations of linear PAHs from naphthalene to hexacene, 118 as well as two additional sets of Loos, Jacquemin, and co-workers. 116 , 117 Our numerical results show that the new RS-DH functionals are the most robust ones among the density functional approximations studied, yielding reliable excitation energies for all kinds of singlet excitations.…”