Ab initio treatment of (H2O)2- and (H2O)6-

MP2 level investigations of (H2O)(6)(-) are reported which have been carried out to assign its vertical electron detachment (VDE) spectrum. Extended basis sets including a floating center are employed which yield an accuracy of a few 0.01 eV for the VDE of (H2O)(2)(-) and (H2O)(3)(-) used as test cases. Results for (H2O)(6)(-) differ considerably from previous treatments and lead to a new assignment of the VDE spectrum. It also turns out that (H2O)(6)(-) is probably adiabatically unstable since total energies are marginally higher than for the ground state of the neutral species in the most stable geometries.