Electronic structures and spectroscopic properties of dimers Cu2, Ag2, and Au2 have been studied by density functional theory (DFT). The relativistic effects increase with the increment in electronic number. Zero order regular approximation (ZORA) relativistic correlation gives reliable parameters of electronic structures. By applying B3LYP as the exchange-correlation functional, a same sequence of orbital levels is obtained for Cu2, Ag2, and Au2. The fully filled d-electrons are not completely inert. The contributions of d-electrons to the frontier molecular orbitals are different in these three molecules. A strong interaction between d- and s-electrons causes an up-shift of d-type and a down-shift of s-type molecular orbitals. For Cu2 and Au2, the separation in d- and s-type molecular orbitals is close and small, which account for the much similar characteristics of the excitation states. For Ag2, a big separation in d- and s-type molecular orbitals is shown owing to the extremely small contribution of d electrons to the frontier molecular orbitals. By TDDFT and DSCF, the spectroscopic terms are assigned and explained in terms of the contributions of s- and d-electrons on the excitations. The prediction based on the present calculation matches well with the experimental data.
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