Several quantum chemical methods including DFT (B3LYP, BP86 functional), coupled-cluster theory (RCCSD(T)), and complete active space multi configurational methods (CASSCF/CASPT2) were used to study the geometric and electronic structures of the scandium disilicon cluster in both neutral and anionic states, ScSi2-/0. On the basis of the computed ground and lower-lying electronic states, and ionization energies of the anion, all the experimental bands in the anion photoelectron spectrum of ScSi2- can now fully be elucidated. The B-3(2) and B-2(2) states are determined to be the ground states of the anionic and the neutral triatomic species, respectively. The transition B-3(2) -> B-2(2) is thus assigned to be responsible for the X band in the photoelectron spectrum. The (2)A(1) neutral state is the final state corresponding to the A band. Although the first two bands arise from ionizations of scandium's 4s and 3d orbitals, all three remaining bands with higher ionization energies are the results of one-electron removals from the Sit moiety orbitals of the anionic ground state B-3(2). Two electronic states of the same representations 1(4)B(2) and 2(4)B(2) are ascribed to be the carriers of the B and C bands, whereas the excited state (4)A(2) is attributed to the last band D of the experimental photoelectron spectrum of ScSi2-. From all accessible vibrations of the ground and excited states computed at the B3LYP level, a simulation of band progressions in the photoelectron spectrum was also carried out and used to provide more insights into the experimental bands.