Most small dianions known in the solid state and solutions cannot exist as isolated entities and decay in the gas phase by electron autodetachment. These dianions show rare-gas-like closed-shell electronic ground states and represent a new type of metastable system. Here we study the prototype closed-shell resonance C-2(2-) in the framework of the complex absorbing potential method. We investigate in detail a number of unsettled methodological issues. In particular, there is no "natural" choice of orbital set for closed-shell metastable states and we study several orbital sets as well as other basis set and correlation effects on resonance energy and width. Closed-shell resonances typically show several open decay channels and we compute partial widths for the three open channels of C-2(2-). Finally, we study the complex potential energy curve and compare our bond lengths and vibrational frequencies with geometrical parameters which have been obtained ignoring the metastable character of C-2(2-). (C) 2000 American Institute of Physics. [S0021- 9606(00)30810-8].