The structural and electronic properties of the carbonyl sulfide dimer anion are calculated using density functional theory within a pseudopotential method. Three geometries are optimized and investigated: C-2v ,and C-2 symmetric, as well as one asymmetric structure. A distribution of an excess charge in three isomers are studied by the Hirshfeld method. In an asymmetric (OCS)(2)(-) isomer the charge is not equally divided between the two moieties, but it is distributed as OCS-0.6 . OCS-0.4. Low-lying excitation levels of three isomers are compared using the time-dependent density functional theory in the Casida approach. (c) 2007 Elsevier B.V. All rights reserved.