In this paper, benchmark results are presented on Coupled Cluster calculation of singlet excitation energies and the corresponding oscillator strength. The test set of Thiel et al. (Schreiber, M.; Silva, M. R. J.; Sauer, S. P. A.; Thiel, W. J. Chem. Phys. 2008, 128, 134110) has been used, and the earlier results have been extended by CC3 oscillator strength for the whole set, CC3 excitation energies for larger molecules, and CCSDT results for some small molecules. Accuracy of the members of the hierarchy CC2-CCSD-CC3-CCSDT has been analyzed. The results show that both CC2 and CCSD are quite accurate and the difference to CC3 excitations energies is typically not larger than 0.2-0.3 eV. While the mean deviation of the CC2 results is close to zero, CCSD systematically overshoots the CC3 results by about 0.2 eV. The standard deviation is, however, somewhat smaller for CCSD, that is, the latter method provides more systematic results. Still, only a few cases could be identified were the absolute value of the error is over 0.3 eV in case of CC2. The results are even better for CCSD, with the exception of uracil, where surprisingly large error of the excitation energies have been found for two of the four lowest n-π* transitions. Both LR (Linear Response) and EOM (Equation of Motion) style oscillator strengths have been calculated. The former is more accurate at both CC2 and CCSD levels, but the difference between them is only 1-2% in case of CCSD. The error of the CC2 oscillator strength are substantially larger than that of CCSD but qualitatively still correct.

We present a comprehensive statistical analysis on the accuracy of various excited state Coupled Cluster methods, accentuating the effect of diffuse basis sets on vertical excitation energies of valence and Rydberg-type states. Many popular approximate doubles and triples methods are benchmarked with basis sets up to aug-cc-pVTZ, with high level EOM-CCSDT results used as reference. The results reveal a serious deficiency of CC2 linear response and CIS(D) techniques in the description of Rydberg states, a feature not shown by the EOM-CCSD(2) and EOM-CCSD variants. The CC3 theory proves to be an accurate choice among the iterative approximate triples methods, while the novel perturbation-based CCSD(T)(a)* variant turns out to be the best way to include the effect of triple excitations in a noniterative way.

In this work the interaction between alkali metal ions and graphene surface with the absence and the presence of external electric field applied perpendicular to the surface was investigated. M05-2X/6-31G(d) DFT calculations were performed to describe the adsorption properties. Results show that the electric field pushes closer the positively charged ion to the graphene, where the charge transfer between the alkali metal cations and the electron rich graphene surface increases. At a species-dependent certain strength of the electric field the excess electrons cause negative charge on the alkali metal ion. This effect will promote the removal of the ion from the surface.

In this paper, coupled cluster methods CC2, CCSD, CCSDR(3) and EOM-CCSD(T) have been benchmarked against CC3 for the transition energies of nucleobases. Beside presenting vertical excitation energies for about 30 singlet transitions of four molecules, the results are analyzed statistically and problematic cases have been discussed in detail. It is concluded that the mean deviation of the CC2 results is smaller than that of the CCSD. However, the latter seems to be more systematic, i.e. it usually overestimates excitation energies by about 0.2 eV but with somewhat smaller standard deviation. Unfortunately, with decreasing single excitation contribution in the wave function CCSD gives large error, which can not be corrected by the non-iterative triples methods CCSDR(3) and EOM-CCSD(T).