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.
Highlights
Charge transfer (CT) states are special types of electronically excited states that play a key role in processes related to molecular conductance and electron transfer properties
Our proposal includes 14 CT states of nine bimolecular complexes and reference values obtained at the CCSDT-3 level with the cc-pVDZ basis set
For the half of these states CCSDT results are available, which show that CCSDT-3 is very accurate, and the approximate treatment of connected triple excitations does not change the quality of the benchmark values
Summary
Charge transfer (CT) states are special types of electronically excited states that play a key role in processes related to molecular conductance and electron transfer properties. Popular approaches based on time-dependent density functional theory (TDDFT) are known to underestimate considerably the excitation energies of CT states, at least when standard functionals are employed.[1] It was shown, e.g., in refs 1−4 that most functionals do not perform well for valence and CT type states at the same time, and only hybrid functionals which include a substantial amount of Hartree−Fock exchange (often close to 100%) are capable of giving reasonable results for CT states.[4] Better performance is observed with long-range corrected (LRC) hybrid models.[1,3,5,6] these functionals were designed to provide the correct long-distance charge transfer behavior,[3] this is essentially achieved via the inclusion of exact exchange.[7] At the same time, LRCs introduce instability issues for triplet states,[8] and the range separation parameter turns out to be system dependent. Recent studies found the restricted open-shell Kohn−Sham approach a promising way of obtaining CT states by DFT methods.[11,12] For more recent developments in the treatment of charge transfer states within TDDFT, the reader is referred to two excellent reviews on the subject.[13,14] Due to these uncertainties, there is still a demand for wave function methods which are economic enough to treat large systems of chemical interest or at least can provide reliable benchmark results to test and calibrate lower level methods
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