An interpretation of the absorption and emission spectra of the gold dimer using modern theoretical tools

The excited states of the gold dimer have been investigated using modern theoretical tools including the multiconfigurational exact molecular mean-field intermediate Hamiltonian Fock-space Coupled Cluster, X2Cmmf-IHFSCC, and the complete active space self-consistent field followed by second order perturbation theory, CASSCF/CASPT2. The computed optically active transitions have been benchmarked against the available experimental data and compared with time-dependent density functional theory, TDDFT, results, both in the two-and four-component schemes. We explored in great detail several spectroscopic properties such as bond lengths, potential energy surfaces (PES), vibrational frequencies and vibrational progressions of the ground and low-lying excited states. Our data show excellent agreement with the experimental measurements and present a significant improvement compared to previous ab initio calculations. They also permit a detailed investigation of the intriguing a <- X and A' <- X experimental bands that, according to our calculations, show an avoided energy level crossing. The location of this crossing is critical for a correct estimation of the vibrational progression and oscillator strengths of these two states. Moreover, among the exchange-correlation (xc) potentials, the SAOP gives the best excitation energies, followed by the hybrid B3LYP functional. Pure functionals like BLYP give by far the worst results.