The charge‐transfer states in a stacked nucleobase dimer complex: A benchmark study

Abstract

Electronic singlet excitations of stacked adenine-thymine (AT) and guanine-cytosine (GC) complexes have been investigated with respect to local excitation and charge-transfer (CT) characters. Potential energy curves for rigid displacement of the nucleobases have been computed to establish the distance dependence of the CT states. The second-order algebraic diagrammatic construction [ADC(2)] method served as reference approach for comparison to a selected set of density functionals used within the time-dependent density functional theory (TD-DFT). Particular attention was dedicated to the performance of the recently developed family of M06 functionals. The calculations for the stacked complexes show that at the ADC(2) level, the lowest CT state is S(6) for the AT and as S(4) for the GC pair. At the reference geometry, the actual charge transferred is found to be 0.73 e for AT. In case of GC, this amount is much smaller (0.17 e). With increasing separation of the two nucleobases, the CT state is strongly destabilized. The M06-2X version provides a relatively good reproduction of the ADC(2) results. It avoids the serious overstabilization and overcrowding of the spectrum found with the B3LYP functional. On the other hand, M06-HF destabilizes the CT state too strongly. TD-DFT/M06-2X calculations in solution (heptane, isoquinoline, and water) using the polarizable continuum model show a stabilization of the CT state and an increase in CT character with increasing polarity of the solvent.

Exciton delocalization, charge transfer, and electronic coupling for singlet excitation energy transfer between stacked nucleobases in DNA: An MS-CASPT2 study

“…Our results for AT and GC differ from previous calculations at different levels of theory for the same dimers in the B-DNA conformation, with a somewhat shorter stacking distance (3.38 Å here versus 3.15 Å in the preceding references). 45,46 In those cases, no mixing between the charge transfer and the excitonic states is reported. For example, at the EOM-CCSD(T) level of theory, the charge transfer state lies approximately 0.4 eV and 0.6 eV above the exciton states for GC and AT, respectively.…”

“…Such charge transfer states have been invoked in previous experimental and computational studies on DNA spectroscopy. 17,22,24,26,27,45 In the stacks AT, GC, and GT, there is a low-lying charge transfer state that mixes with the two locally excited diabatic states. In the AC dimer, the charge transfer character is not so prominent (8% for S 2 ).…”

Assessment of a composite CC2/DFT procedure for calculating 0–0 excitation energies of organic molecules

“…In terms of MAEs, the best-performing M06-HF through the inclusion of full HF exchange is more broadly applicable than B3LYP and PBE0 to different types of excited states -including Rydberg and charge-transfer states [77,78] that pose a considerable challenge to TD-DFT [52,55,[105][106][107][108]] -this functional is by some margin the least accurate functional for the current benchmark set of low-lying organic valence states. Since the MAE for M06-HF is 0.21 eV larger than the MAE for M06-2X (with 54% HF exchange), which in turn is 0.12-0.16 eV larger than the MAEs for PBE0 (25%) and B3LYP…”

Computational Modeling of Photoexcitation in DNA Single and Double Strands

“…Matsika and coworkers [61] compared the performance of different methods (including CIS, TDDFT, CASSCF, and CC) in the description of excited-state π-stacked nucleobases. A benchmark study by Aquino et al [130] reported stable interbase charge-transfer states for stacked adenine-thymine and stacked guanine-cytosine, the simplest stacked base pairs. However, the nature, and especially the delocalization degree, of the initially populated excited states in real DNA systems remains a puzzle [37].…”

“…In comparisons [122] of three recent LRC functionals, namely BNL [123,124], CAM-B3LYP [125], and LC-PBE0 [126,127], it was found that only CAM-B3LYP gave reasonable energies for the interbase charge-transfer excited states of the hydrogen-bonded Watson-Crick A·T and G·C base pairs. There are also indications that the meta-hybrid M06-HF [128] and M06-2X [129] functionals may be adequate to treat the photoexcitation of nucleobase monomers and oligomers [130]. However, in a systematic excited-state dynamics study of 9H-adenine (Ade), the experimentally observed ultrafast decay was not reproduced in TDDFT-based surface hopping simulations with any of the six tested functionals (PBE, B3LYP, PBE0, CAM-B3LYP, BHLYP, and M06-HF) whereas reasonable decay times were obtained at the ab initio MRCIS and the semiempirical OM2/MRCI levels [131].…”