A TD-DFT basis set and density functional assessment for the calculation of electronic excitation energies of fluorene

Conjugated organic materials are the subject of intensive research for a range of optoelectronic applications. A model for such molecules is fluorene, which consists of rigid planar biphenyl units of C_2v symmetry. A low energy experimental absorption spectrum in the gas phase is composed of A ₁ and B₂ transitions. The aim of this work is to evaluate the performance of the basis sets cc-pVXZ (X = D and T), aug-cc-pVDZ, 6-31G**, 6-31++G**, 6-311G**, 6-311++G**, Sadlej-pVTZ, Z2Pol, Z3Pol, and pSBKJC and of the functionals B3LYP, B3LYP/CS00, CAM-B3LYP, PBE0, and LB94 in predicting the electronic transitions obtained taking linear response-coupled cluster singles and doubles (LR-CCSD) results as the theoretical reference. Our findings suggest that the time-dependent density functional theory singles method is not able to correctly assign the predicted spectrum while LR-CCSD always correctly describes the experimental data. Among the studied density functionals, the best performance was achieved by the CAMB3LYP. For transitions above 5 eV, diffuse functions are required to properly predict the observed transitions.