Noncovalently bound excited-state dimers: a perspective on current time-dependent Density Functional Theory approaches applied to aromatic excimer models

Abstract

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for time-dependent Density Functional Theory (TD- DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well- developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrid—particularly with large fractions of Fock exchange—are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

Definition and benchmarking of ab initio fragment methods for accurate excimer potential energy surfaces

“…Note that similar developments are needed for TD-DFT methods to include state-specific dispersion interaction. 62 Third, to also reliably predict equilibrium structures of these complexes, the inclusion of the Dexter coupling is necessary. At present, the highest-level wavefunction method available for this purpose is CC2, 66 but our results (discussed in the Supplementary material of this paper) show that the transition properties predicted by this method can be very different from those obtained with CCSD (see also Ref.…”

“…Note that this is a common practice in TD-DFT studies on excited states of non-covalent complexes. 62 The hope is that the error made this way is significantly smaller than the one completely neglecting these terms would introduce. This idea is supported by the fact that in excited states dominated by a single substitution, only a small fraction of the electrons are affected by the excitation and the dominant part of the electron density is similar in the ground and excited states.…”

“…To find the approximation most suitable for excited states of non-covalently interacting dimers of nucleobases, we investigate, as in our previous paper on the ground state, 21 The performance of methods describing excitonic states, as it has also been pointed out recently by Hancock and Goerigk, 62 should not just be evaluated in single point calculations.…”