Excited states of aurocarbons: CASPT2 and CCSD(T) calculations of C2Au2 and C2Au4

We present CASPT2 calculations of vertical excitation energies for low-lying singlet and triplet states of auroderivatives of acetylene and ethylene representing small model aurocarbons. Data are supplemented by CCSD(T) results for triplet states. All four considered species, namely linear C2Au2 molecule and three conformers of the C2Au4 molecule-Au2C2Au2 (tetraauroethylene, the analog of the parent ethylene molecule) and s- and p-adducts of the Au-2 molecule with the auroacetylene exhibit considerable lowering of low-lying excitation energies when compared with their parent acetylene and ethylene molecules. Singlet and triplet excitation energies of diauroacetylene drop by 57% and 48%, respectively, and of tetraauroethylene, by 68% and 56% when compared with their respective parent molecules. Even more is lowered the singlet-triplet energy gap. We stress the importance of the dynamical correlation in CASPT2 calculations and discuss problems with selection of the appropriate active space in aurocarbons.