Valence and charge-transfer optical properties for some Si_nC_m (m, n ≤ 12) clusters: Comparing TD-DFT,...

Accurate optical characterization of the closo-Si₁₂C₁₂ molecule is important to guide experimental efforts toward the synthesis of nano-wires, cyclic nano-arrays, and related array structures, which are anticipated to be robust and efficient exciton materials for opto-electronic devices. Working toward calibrated methods for the description of closo-Si₁₂C₁₂ oligomers, various electronic structure approaches are evaluated for their ability to reproduce measured optical transitions of the SiC₂, Si₂C_n (n = 1-3), and Si₃C_n (n = 1, 2) clusters reported earlier by Steglich and Maier [Astrophys. J. 801, 119 (2015)]. Complete-basis-limit equation-of-motion coupled-cluster (EOMCC) results are presented and a comparison is made between perturbative and renormalized non-iterative triples corrections. The effect of adding a renormalized correction for quadruples is also tested. Benchmark test sets derived from both measurement and high-level EOMCC calculations are then used to evaluate the performance of a variety of density functionals within the time-dependent density functional theory (TD-DFT) framework. The best-performing functionals are subsequently applied to predict valence TD-DFT excitation energies for the lowest-energy isomers of Si_nC and Si_n-1C_7-n (n = 4-6). TD-DFT approaches are then applied to the Si_nC_n (n = 4-12) clusters and unique spectroscopic signatures of closo-Si₁₂C₁₂ are discussed. Finally, various long-range corrected density functionals, including those from the CAM-QTP family, are applied to a charge-transfer excitation in a cyclic (Si₄C₄)₄ oligomer. Approaches for gauging the extent of charge-transfer character are also tested and EOMCC results are used to benchmark functionals and make recommendations.